Assessing Urban Chromatic Contagion: A Quantitative Index and an Epidemiological Approach to Prevent Visually Disruptive Facade Interventions

Abstract

Façades play a decisive role in shaping the visual and symbolic character of historic urban environments. Recent European funding schemes promoting energy-efficient retrofitting have accelerated interventions on building envelopes. Although aligned with decarbonization objectives, these processes are generating significant chromatic and material transformations that risk eroding the visual coherence and cultural sustainability of consolidated urban areas. In the historic Ensanches of San Sebastián, the replacement of traditional envelope systems with new cladding solutions is leading to the loss of the architectural style of some facades and altering their materials, textures, and colors. A progressive “contagion effect” has been identified, whereby dissonant chromatic schemes—often associated with the proliferation of so-called “zebra blocks”, residential buildings with façades clad in alternating black and white stripes that have proliferated in recent urban developments—are replicated across adjacent buildings, gradually weakening spatial continuity and the genius loci of the neighborhood. In response to this phenomenon, this research develops a systematic methodology to analyze, quantify, and anticipate chromatic transformation in consolidated urban fabrics. The study combines historical morphological analysis, classification of architectural periods, and chromatic mapping of recent façade interventions. Based on this framework, a CARI, Chromatic Alteration Risk Index is proposed to evaluate the potential impact of façade alterations on urban chromatic coherence. Drawing on an epidemiological framework, the methodology enables the identification of critical transformation clusters, the assessment of contagion dynamics, and the definition of regulatory thresholds for color and material interventions. By integrating perceptual criteria, urban morphology, and spatial distribution patterns, the study moves beyond descriptive diagnosis and offers a transferable tool for municipal planning. The proposed approach supports the proactive regulation of façade rehabilitation processes, balancing energy efficiency objectives with the preservation of collective memory, material identity, and urban sensory quality. This study proposes a quantitative model of “urban chromatic contagion” to assess how façade color interventions propagate within a neighborhood. We define the Chromatic Integration Percentage (CIP) and the Chromatic Alteration Risk Index (CARI) of the analyzed area. Results indicate that poorly regulated façades show higher chromatic dissonance (low CIP) and act as contagion hotspots, while a clear risk gradient emerges: highly protected buildings present lower risk, whereas mixed typologies and recent rehabilitations concentrate higher CARI values. The model supports preventive urban color management by identifying areas at risk before visible alteration.

1. Introduction

Façades constitute an essential component in shaping urban character, as they integrate practical functions, aesthetic values, and representational meaning within the urban fabric, particularly in historic centers. Their relevance extends beyond the merely physical dimension, encompassing perceptual, symbolic, and emotional aspects [1] that mediate between the private realm of the building and the collective experience of urban space [2]. In architectural and urban analysis, the façade thus emerges as one of the principal supports of visual identity and the historical recognition of the built environment [3,4].

In recent years, interventions on building envelopes have increased significantly in Spain, primarily driven by energy retrofitting programs associated with European funding schemes [5,6,7]. These actions are essential for advancing decarbonization targets and improving the thermal performance of the existing building stock. However, when implemented in consolidated urban fabrics with historical value, they generate impacts that extend beyond the individual building and affect the urban image as a whole, particularly when they are not based on clear criteria regarding façade composition, materials, texture, and color scheme (Figure 1) [8].

In this context, façade color assumes a central role [9]. Beyond a purely aesthetic consideration, chromatic expression constitutes a key factor in the perception of urban space [10], visual comfort [11], and the construction of the collective imaginary associated with a place [12].

In addition to the well-documented influence of color on individuals’ psychological processes through visual perception [13], numerous studies in environmental psychology and urban design have shown that chromatic coherence contributes to perceptual stability and environmental legibility, whereas repeated dissonances may lead to identity loss and visual homogenization. Color therefore shapes citizens’ perceptions of urban space and may alter the emotional bond between inhabitants and the city [14,15].

Nasar (1994, 1999) [16] demonstrated that the aesthetic quality of urban scenes depends on the formal and chromatic coherence of façades [17]. The diffusion of color in urban contexts has been addressed in parametric urban design studies, where it is observed that emphasized chromatic interventions can influence the perception of entire streets. In particular, the concept of “visual contagion” appears in theories of social dynamics applied to urban space, where striking changes (such as intense color) in one building can trigger reactions in neighboring buildings (through imitation or contrast) [18,19]. In this study, we define urban chromatic contagion as the process by which a significant chromatic intervention on a façade tends to visually propagate to neighboring façades through mechanisms of imitation or perceptual similarity. Thus, chromatic contagion is situated within the literature on chromatic parameters in façades and processes of urban harmony [20].

1.1. Quantitative Assessment of Urban Color and Limitations of Current Approaches

Recent international research has made significant progress [21] in the quantitative assessment of urban color [22], particularly through the development of indices of chromatic harmony, diversity, and coherence applied to façades and urban landscapes [23]. These approaches typically rely on the identification of dominant colors and their statistical analysis, with the aim of characterizing the chromatic structure of streets, neighborhoods, or historic districts and comparing different urban environments [24,25].

In parallel, advances in computer vision and the availability of street-level imagery have fostered research that uses Street View data and multi-scalar analysis [26,27,28,29], enabling large-scale, automated assessments of urban color [30]. These methods have proven effective in detecting general chromatic patterns and analyzing extensive built environments at relatively low cost. Moreover, a significant portion of this literature has incorporated perceptual validation processes [31], combining objective color metrics with subjective evaluations, visual comfort studies, and even eye-tracking techniques [32], thereby strengthening the empirical relationship between color, perception, and urban experience.

However, despite these advances, existing approaches present significant limitations when applied to consolidated urban fabrics with historical value, particularly in the current context of intensive energy retrofitting. First, many of these methods are predominantly descriptive in nature, focusing on assessing the chromatic condition of an environment at a specific point in time, without incorporating tools capable of anticipating future transformation processes or identifying areas or buildings more vulnerable to cumulative chromatic alterations.

Second, methods based exclusively on street-level imagery present well-documented technical constraints stemming from lighting variability, the presence of shadows, and the chromatic fidelity of images, which limit their suitability for defining precise reference palettes. In historic environments, where chromatic coherence is closely linked to specific materials, construction techniques, and aging processes, these limitations become particularly significant.

Furthermore, a substantial portion of the literature on chromatic harmony and diversity focuses on abstract visual metrics [33], without systematically integrating key variables for urban management, such as façade materiality [34], the level of heritage protection, or the conservation status of buildings [35]. This disconnect hinders the translation of findings into operational criteria applicable to urban planning, regulatory frameworks, or permitting processes, particularly in historic neighborhoods subject to intense transformation pressures [36].

1.2. Contribution of the Study: From Chromatic Measurement to Preventive Management

In contrast to these approaches, the present study proposes a complementary framework specifically oriented toward the preventive management of urban color in consolidated urban fabrics [37]. In this context, there is a need for operational tools capable of translating the theoretical principles of urban chromatic identity into comparable and replicable analytical criteria. To this end, the study introduces a set of indices aimed at the quantitative assessment of façade color, conceived as decision-support instruments for intervention processes in consolidated urban areas.

Unlike approaches that rely solely on perceptual judgments or general chromatic evaluations, the proposed indices ground the assessment in the relationship between built color and its immediate context, incorporating parameters derived from colorimetric analysis of the surrounding environment.

The Chromatic Integration Percentage (CIP) evaluates the compatibility of façade colors with a local baseline palette defined through the colorimetric analysis of representative materials within the surrounding context. In this way, the indicator directly reflects the neighborhood’s chromatic identity, enabling comparisons across buildings supporting the assessment of changes introduced through rehabilitation processes.

Similarly, the Chromatic Alteration Risk Index (CARI) introduces a dimension largely absent from the existing literature: the anticipation of future alteration risk. Unlike previously published indices of chromatic harmony or coherence, CARI integrates chromatic, constructive, and regulatory variables—façade material, level of protection, presence of already altered adjacent buildings, and conservation status—in order to estimate the probability of transformation and to identify spatial hotspots and gradients of risk. This approach makes it possible to interpret chromatic alteration as a cumulative and spatially driven process, rather than as the sum of isolated interventions.

In this regard, the proposed methodology does not replace existing international approaches but rather complements them [4], addressing a specific gap between the measurement of urban color and its effective application in rehabilitation policies, historic urban landscape conservation, and cultural sustainability [38]. Its scope is clearly transferable to other urban extensions and consolidated neighborhoods subject to similar dynamics of energy renovation and visual transformation [39].

2. Objective

The objective of this research is to develop and propose a systematic methodology to identify and quantify dissonant chromatic alterations generated by façade rehabilitation interventions. The methodology aims to objectively quantify the degree of chromatic integration of façades within their urban context, assessing the visual coherence of individual buildings [9] as well as of the ensembles in which they are embedded.

Furthermore, it aims to determine a risk index—referred to as the “Chromatic Alteration Risk”—for buildings susceptible to future rehabilitation, taking into account the historical evolution of chromaticity within the neighborhood. The methodology is conceived to be tested and validated [40] by experts in architecture and urban planning, ensuring its technical rigor, and to be designed in a replicable manner so that it can be applied across different urban contexts using consistent and comparable criteria.

3. Materials and Methods

The present research proceeded in several phases, to analyze and understand chromatic alterations in building façades in Donostia’s historic center following architectural interventions. Drawing on the framework of urban chromatic epidemiology, the methodology combines visual analysis techniques, typological classification, and documentary review, enabling a comprehensive assessment of the phenomenon under study (Figure 2).

3.1. Detailed Building Analysis and Classification

First, a detailed analysis of each building identified within the neighborhood is conducted, focusing on aspects such as materiality (types of materials used in façades), composition (distribution and organization of architectural elements), and color (original tonalities). This process involves on-site visits complemented by photographic records and standardized characterization sheets. The analysis makes it possible to determine the specific constructive features of each building and to facilitate its classification into different typologies.

This classification relies on a comparative analysis of constructive, ornamental, and chromatic elements to identify patterns and relationships across typologies. In this sense, typological categories are defined through the recognition of recurring combinations of construction systems, materials, textures, and color schemes, which together characterize coherent architectural families. Rather than being based on isolated features, the classification considers the interrelation between these variables, allowing each building to be understood as part of a broader system of shared attributes. This approach supports a more precise identification of the buildings and provides a structured framework for the subsequent chromatic analysis, as color is interpreted in relation to the material and constructive context in which it appears. Consequently, the typological grouping not only organizes the architectural diversity of the area but also reveals consistent chromatic patterns associated with specific constructive and material conditions.

3.2. Determination of Traditional Colors

3.2.1. Study of Original Materials and Colors

The study of original materials and colors constitutes an essential phase in any chromatic façade rehabilitation intervention [34]. Its purpose is to identify the specific chromatic identity of an individual building, of buildings belonging to the same architectural style, or of their immediate urban context, in order to establish objective criteria for conservation, integration, and the proposal of new finishes.

To this end, a general methodology structured in two complementary phases is proposed. The first phase corresponds to fieldwork, which includes the identification of buildings and of the specific elements under study—such as main façades, visible party walls, singular elements, joinery, string courses, or plinths—as well as the recognition of existing materials, which may include stone, render, exposed brick, wood, or metal. During this phase, chromatic data are also collected in situ using a colorimeter or other direct measurement instrument.

The second phase focuses on the processing and synthesis of the collected information. This involves transferring all measurements into comparative records associated with façade elevations or schematic drawings, providing a precise description of materials, and rigorously identifying and coding colors using a standardized color system. Finally, individualized chromatic palettes are generated for each building [9], followed by a comparison between the current condition and the chromatic configurations prior to interventions—when such information is available—as well as a systematic comparison among surrounding buildings, with the aim of assessing coherences, divergences, and shared patterns.

In this way, each sheet or record compiles the complete chromatic study: material composition, measured colors, and their translation into a defined chromatic palette. This facilitates comparative analysis and the identification of chromatic coherences and dissonances within the urban ensemble.

For color-coding, the use of the HSL model (Hue, Saturation, Lightness) [41,42] is proposed, selected for its intuitive structure and its close alignment with human visual perception. This system describes color through three fundamental parameters. Hue uses degrees from 0° to 360° on a chromatic circle, where each value corresponds to a spectral color (e.g., red at 0°/360°, green at 120°, and blue at 240°). Saturation uses a percentage and reflects color intensity: higher values yield more vivid tones, whereas lower values produce desaturated or pastel colors (Figure 3). Lightness also uses a percentage to indicate how bright or dark a color appears, ranging from high values for luminous tones to low values for muted shades [15].

Unlike RGB, which relates to light-emitting devices, or CMYK, which supports printing and subtractive color mixing, the HSL model better supports intuitive color interpretation. It also enables precise adjustments in design, rehabilitation, and chromatic harmonization contexts. In addition, HSL facilitates communication among technical professionals—architects, urban planners, and designers—and external stakeholders such as clients and public administrations. Consequently, most experimental studies on harmonious color planning treat hue, saturation, and brightness as the most relevant attributes [43,44,45].

While CIELab is widely recognized for its perceptual uniformity and precision in color measurement, the choice of HLS in this study responds to its suitability for qualitative and comparative analysis of urban color in architectural contexts.

HLS offers an intuitive framework based on hue, lightness, and saturation—dimensions that closely align with how color is perceived and described in architectural practice. This is particularly relevant in urban and heritage studies, where color must be interpreted in relation to materials, textures, and construction systems, rather than only measured [46].

Additionally, HLS (and related models such as HSV) has been effectively applied in recent image-based analyses of urban façades, enabling the identification of dominant hues and chromatic distributions while maintaining a direct link to perceptual and design-oriented interpretations.

Although CIELab provides higher colorimetric accuracy, its parameters (L*, a*, b*) are less intuitive for architectural analysis and can limit the readability of typological comparisons. In contrast, HLS facilitates clearer interpretation of façade color variations, especially those influenced by material properties and surface finishes.

Therefore, HLS is considered more appropriate for this research, as it supports an accessible, consistent, and typologically oriented approach to the analysis of urban color.

3.2.2. Color Study Procedure Using a Colorimeter

Color constitutes a physiological perception, and individual observers therefore experience it differently. Nevertheless, researchers can obtain an objective assessment through colorimetric analysis [47]. To identify original colors, this study employs a portable colorimeter (model PCE-XXM 30).

The recommended measurement procedure, designed to be replicable in any building or urban context, follows a clearly defined operational sequence. First, the study surfaces are selected by identifying areas representative of each material and finish—such as primary wall planes, ornamental elements, or joinery—while avoiding, as far as possible, zones affected by localized dirt, stains, recent repainting, or severe degradation. Subsequently, the colorimeter is prepared and positioned perpendicular to the surface, ensuring the material fully covers the measurement aperture to minimize interference from ambient light.

For each sample, researchers take multiple measurements and record at least four readings from different points on the same surface. They document all values. This repeated sampling captures pigmentation variation and reduces errors associated with isolated measurements. The team processes the data using the colorimeter’s software and establishes a reference sample for each surface or material. The software then compares the remaining measurements against that reference using evaluation functions such as “PASS,” which indicates that the sample falls within the predefined chromatic tolerance range, and “FAIL,” which indicates that the sample deviates excessively from the reference. To ensure the reliability and comparability of the measurements, particular attention must be paid to environmental conditions during data acquisition. Although the colorimeter incorporates its own controlled light source, measurements should be conducted in a manner that minimizes the influence of external illumination. Direct solar radiation, in particular, may introduce stray light or specular reflections, especially on slightly textured or semi-gloss painted surfaces, potentially affecting the accuracy of the readings. For this reason, measurements are preferably carried out under diffuse lighting conditions—such as in shaded areas or during periods of low solar incidence—and always ensuring full contact between the device and the surface. By maintaining consistent measurement conditions and avoiding surfaces affected by moisture or extreme temperatures, researchers can further enhance the repeatability and robustness of the collected chromatic data.

Finally, the researchers determine each surface’s representative color by selecting either the most frequently recurring measurement or the value that the software identifies as closest to the reference sample within the established tolerance limits. They then report this color in the adopted reference system, which in this methodology corresponds to the HSL model.

In addition, the researchers empirically verify the recorded color by visually comparing it with standardized NCS color charts. This step allows for the perceptual validation of the instrumental results and facilitates their interpretation and communication within the architectural design process.

Researchers can systematically replicate this procedure for any building or group of buildings, thereby producing a coherent, comparable chromatic database.

3.3. Identification of Chromatic Alterations

To identify buildings that have altered their original color, the researchers reviewed historical and current imagery. They used Google Maps 26.xx because it offers a longitudinal, regularly updated record of façade conditions. The team analyzed available images to document pre-intervention colors and track changes over time. In addition, they compiled the year of each intervention, and the colors applied by reviewing municipal reports and photographic records of the works.

Based on the collected information, the researchers established a chronological record of chromatic alterations, which allowed them to identify trends and patterns in the evolution of façade colors. This approach clarifies how interventions and broader urban transformations influence aesthetic perception and the conservation of architectural heritage.

In addition, the team analyzed historical images available on platforms such as Google Maps (Street View). Although these images do not support precise colorimetric measurement, they help detect rehabilitation processes and significant color changes over time. They also provide an approximate indication of the chromatic range prior to interventions [48].

3.4. Creation of the Reference Color Palette

The palette is constructed through a controlled restriction of the chromatic space, prioritizing warm and sand tones characteristic of the analyzed context. To this end, the following ranges are defined: Hue (H): between 10° and 40° (within a global range of 0–360°); Saturation (S): between 20% and 70% (within a global range of 0–100%); Lightness (L): between 40% and 90% (within a global range of 0–100%).

These limits allow for a range spanning from slightly tinted whites to semi-saturated colors within the selected spectrum, resulting in a balanced palette that is representative of the environment.

Based on these parameters, a reference chromatic matrix is constructed (Figure 4), which functions as a validation tool. This system enables an objective assessment of whether the colors present in the analyzed samples conform to the defined palette through the direct comparison of their HSL values.

3.5. Verification of Chromatic Compatibility

Once the researchers define the baseline palette, they establish a systematic criterion to evaluate whether a given color—existing or proposed—fits the analyzed urban context. They first represent the color in the HSL model, drawing on colorimetric measurements for existing façade colors or design software for proposed chromatic solutions.

They then compare the color’s hue, saturation, and lightness values with the predefined intervals that define the baseline palette. The criterion classifies a color as compatible only when all three parameters fall within the specified ranges; if any parameter falls outside these thresholds, the researchers classify the color as incompatible.

Calculation of the Chromatic Integration Percentage of a Building

The researchers propose a Chromatic Integration Percentage to quantitatively assess how closely a building aligns with the chromatic palette of its surrounding context. They can apply this tool systematically to any urban ensemble.

To evaluate a building’s chromatic integration, the researchers first identify the colors present on the façade, including wall planes and components such as ornamental elements, plinths, joinery, railings, and other relevant features.

Particular attention is given to the colors of the main façade planes, as these constitute the largest visible surfaces and therefore have the greatest impact on the overall chromatic perception of the building within the urban context. In this sense, chromatic alteration is primarily understood in relation to these dominant surfaces, which define the visual identity of the façade at the urban scale. Secondary elements—such as joinery, railings, or ornamental details—are also recorded and evaluated; however, their influence on the final assessment is considered complementary, as they typically occupy smaller surface areas and have a more limited perceptual impact.

The CIP calculation adopts an equal-weight approach for main façade planes colors in order to ensure methodological simplicity, transparency, and replicability. While this implies a simplification of perceptual hierarchies, it allows for consistent comparison across a large number of buildings and avoids introducing subjective biases associated with estimating surface proportions. Moreover, the explicit identification of dominant façade planes during the analysis phase ensures that the most relevant chromatic features are systematically included in the evaluation.

They measure each color and express it in the HSL model, which enables an individual assessment of its compatibility with the baseline chromatic palette. The team verifies compatibility by checking whether each color’s hue, saturation, and lightness values fall within the predefined ranges. They then assign a binary score: 1 indicates compatibility, and 0 indicates that the color lies outside the established thresholds. Based on this evaluation, the Chromatic Integration Percentage (CIP) of the building is calculated for both its original and current façade, in relation to a previously defined baseline chromatic palette using HSL codes. This indicator enables comparisons among different buildings, the assessment of chromatic evolution before and after rehabilitation interventions, and the establishment of minimum integration thresholds within regulatory frameworks or urban design guidelines.

The evaluation process, focused exclusively on color attributes, assigns one point to each façade color that falls within the predefined ranges of the reference palette. The final score for each building is expressed as a percentage, representing the proportion of compatible colors relative to the total number of identified colors. For example, a building with 7 compatible colors out of 10 identified colors achieves a chromatic integration of 70%.

The compatibility of each color is determined by comparing its Hue, Saturation, and Lightness values with the intervals defined in the baseline palette. These intervals were established through a combined procedure involving: (1) direct colorimetric measurements of representative materials within the study area; (2) statistical analysis of the distribution of HSL values across the sample, allowing the identification of recurring chromatic ranges; and (3) validation through expert judgment, in which specialists reviewed representative and borderline cases to adjust the thresholds. As a result, a color is considered compatible only when its three parameters (H, S, and L) fall within the corresponding predefined intervals.

The objective is not to reconstruct a single, immutable “original color,” but rather to delineate a coherent chromatic framework capable of reflecting the material and perceptual identity of the neighborhood and serving as a reference for evaluating integration and dissonance.

This procedure ensures that the classification is not arbitrary, but grounded in both empirical data and expert consensus, thereby improving the transparency and replicability of the method.

The Chromatic Integration Percentage (CIP) is based on a binary classification (compatible/incompatible), which simplifies the inherently continuous nature of chromatic relationships. This decision responds to the need to operationalize the analysis and to enable systematic comparison across a large number of façades and typologies. While chromatic compatibility can be understood as a gradient, the adoption of a discrete classification allows for clearer interpretation and reproducibility of the results within an urban-scale study.

The threshold for determining compatibility was established through a combined approach integrating quantitative criteria and expert judgment. First, preliminary ranges of chromatic similarity were identified based on the distribution of hue, lightness, and saturation values observed in the sample. These ranges were then evaluated and adjusted through the expert validation process, in which specialists assessed representative cases and discussed borderline situations.

In this context, the binary classification does not aim to fully capture the complexity of chromatic perception, but rather to identify whether a façade can be considered integrated within its chromatic context according to shared typological and material characteristics. The use of a consensus-based threshold (≥80% agreement among experts) ensures that the decision rule is not arbitrary, but grounded in collective professional judgment.

Therefore, although simplified, the CIP provides a robust and replicable indicator for assessing chromatic integration at the urban scale, balancing analytical clarity with perceptual relevance.

3.6. Methodology for Calculating the Chromatic Alteration Risk Index of Façades

Within the framework of urban chromatic epidemiology or chromatic contagion, the Chromatic Alteration Risk Index (CARI) is structured based on variables analogous to epidemiological models of risk, exposure, and susceptibility, enabling the quantification of the probability of chromatic alteration for each building within its urban context. To assess the risk of chromatic alteration in building façades, the methodology defines the following factors and assigns scores based on each property’s specific characteristics. The final formula makes it possible to identify the risk index associated with each building as a function of these factors.

The factors considered in calculating the Chromatic Alteration Risk Index comprise four principal dimensions, each associated with a numerical value reflecting its contribution to overall risk. The Material Factor (MF) determines risk according to the predominant façade material, assigning zero points to stone; one point to clad, ventilated façades and lightweight cladding systems (LCS); and two points to ceramic tiling, painted renderings, or combinations of LCS and render in post-war buildings. For façades composed of multiple materials, the method calculates a weighted value based on each material’s proportion, using the expression MF = (weighted sum)/2.

The Level of Protection Factor (LPF) evaluates regulatory protection against colour alterations, assigning two points to unprotected façades, one point to those with intermediate protection (PL2), and zero points to those with high protection (PL1). Current regulations in each city adjust the values, and the calculation then divides the result by two.

The Adjacent Altered Buildings Factor (AABF) considers the influence of neighbouring buildings presenting chromatic alterations, assigning zero points where no altered buildings are present, one point where between one and three are identified, and two points where more than three are observed; the final value is likewise divided by two to weight its effect.

Finally, the Building Conservation State Factor (BCSF) relates the physical condition of the façade to its risk of alteration, assigning zero points to façades in good condition, one point to those with minor localized deterioration, 1.5 points to cases involving constructive deterioration with material detachment, and two points to severe deterioration; this value is also divided by two (

Table 1. Weighting Formulas and Assigned Values for CARI.

Code	Weighting Formula	Factor	Assigned Value
MF	(Σ weighted values of existing materials)/2	Material Factor
		Stone	0
		Metal cladding	1
		Ventilated façade	1
		ETICS	1
		Tiling	2
		Painted render	2
		Combination of ETICS/render in post-war buildings	2
LPF	X/2	Level of Protection Factor
		No protection	2
		PL2 (intermediate protection)	1
		PL1 (high protection)	0
AABF	X/2	Adjacent Altered Buildings Factor
		None	0
		1 to 3 altered buildings	1
		More than 3 altered buildings	2
BCSF	X/2	Building Conservation State Factor
		Good appearance	0
		Minor localized deterioration	1
		Constructive deterioration with detachments	1.5
		Severe deterioration	2

).

$$\mathrm{C}\mathrm{A}\mathrm{R}\mathrm{I}={\displaystyle \frac{\mathrm{M}\mathrm{F}+\mathrm{L}\mathrm{P}\mathrm{F}+\mathrm{A}\mathrm{A}\mathrm{B}\mathrm{F}+\mathrm{B}\mathrm{C}\mathrm{S}\mathrm{F}}{4}}\times 100$$

This index classifies risk into five categories: values between 0% and 10% are considered very low risk; between 10% and 25%, low risk; between 25% and 50%, moderate risk; between 50% and 75%, high risk; and between 75% and 100%, very high risk.

In summary, the proposed methodology is grounded in a series of explicit assumptions necessary to translate a complex and inherently perceptual phenomenon—the chromatic coherence of the urban landscape—into quantifiable and operational indicators at the urban scale. These assumptions do not seek to oversimplify reality, but rather to render it assessable and manageable within urban planning and heritage conservation practice.

With regard to the Chromatic Alteration Risk Index (CARI), the equal weighting assigned to the four factors considered—façade material (MF), level of protection (LPF), presence of already altered adjacent buildings (AABF), and state of conservation (BCSF)—reflects a deliberate methodological decision. In the absence of sufficient empirical evidence to objectively assign differentiated weights, a homogeneous weighting scheme was adopted to avoid introducing arbitrary biases and to facilitate the application of the index across diverse contexts. To evaluate this assumption, a sensitivity analysis was conducted by varying each weight between 0.5 and 2. The main results (spatial patterns of CARI) remain stable, demonstrating the robustness of the methodology under different weighting schemes. This choice reinforces the exploratory and preventive character of the indicator, conceived as a diagnostic and prioritization tool rather than as a deterministic predictive model.

Ultimately, CARI is interpreted as a chromatic vulnerability index, useful for prioritizing façades, but it is not a probabilistic predictive model.

Several elements support the method’s robustness. First, the internal consistency of the results, which reveal spatially coherent patterns—such as the identification of alteration clusters and risk gradients—that are unlikely to be attributable to chance. Second, the convergence among the different CARI factors: the highest values systematically concentrate in buildings characterized by greater material vulnerability, lower regulatory protection, and proximity to prior alterations, thereby reinforcing the logical validity of the index. Finally, the modular structure of the method allows for future adjustment and refinement—whether through recalibration of chromatic ranges, modification of weightings, or incorporation of additional variables—without compromising its underlying conceptual framework.

In this sense, the methodology does not function as a closed system; instead, it provides an open, evolving tool that supports decision-making in urban rehabilitation processes and offers a foundation for future research on the relationship between color, perception, and the cultural sustainability of the urban landscape.

3.7. Expert Validation Procedure

In this phase, 7 experts, architects, historians and urban planners with experience in urban color examined the obtained values. Each expert independently evaluated a sample of representative façades and subjectively classified their “chromatic compatibility” based on their professional judgment.

Subsequently, a Delphi-type protocol was applied to refine and validate these evaluations. The Delphi method is a structured technique designed to achieve consensus among a panel of experts through iterative rounds of assessment, anonymity of responses, and controlled feedback [49,50]. In this study, the first round consisted of the independent evaluation of the façades. The results were then collected, anonymized, and statistically summarized. In a second round, this synthesized information was returned to the experts, who were invited to review and, if necessary, revise their initial judgments in light of the group response. This iterative process reduces the influence of dominant individuals and promotes convergence towards a shared evaluation framework.

Finally, remaining discrepancies were addressed in a guided discussion session to clarify criteria and reinforce consensus. Consensus was defined as agreement among ≥80% of the experts on a given evaluation. This protocol provides additional empirical validation of the indicators.

4. Case Study

4.1. The Gros Neighborhood

San Sebastián’s urban development responded to its complex topography, shaped by the bay, the Urumea River, and the coastal sandy areas. The transformation of this natural setting into urban land formed part of the city’s historical expansion process [51]. The approval of the 1864 Ensanche plan enabled the consolidation of the extension opposite the historic center, linking urban growth to the economic development associated with seaside tourism [52,53].

While the central Ensanche consolidated rapidly, the Gros area remained for decades a peripheral and partly unstable zone, shaped by industrial uses, infrastructure, and working-class housing. The formal urbanization of the neighborhood began with the 1891 plan, which defined the area between the river, the sea, and the railway line. However, private ownership of the land slowed development and produced a less homogeneous urban fabric than in the main Ensanche.

Late nineteenth-century port legislation enabled the drainage of marshlands and the creation of new buildable land through private initiative, thereby consolidating the process of coastal expansion [54,55,56,57]. This growth, interrupted by European and national armed conflicts, resumed in the 1920s and continued throughout the twentieth century in successive construction phases (Figure 5) [58].

From an architectural and chromatic perspective, the neighborhood evolved from an initial functional neoclassical architecture, characterized by the predominant use of local sandstone—an identity-defining element of San Sebastián’s urban landscape [59]—toward more eclectic, rationalist, and developmentalist languages. Over time, builders introduced new materials—reinforced concrete, exposed brick, painted render, and stone cladding—thereby generating substantial formal diversity.

Despite this stylistic heterogeneity, the current result is a high-density urban fabric in which a relatively coherent chromatic palette predominates (Figure 6), based on sandy tones, reddish brick hues, and earthy renders, thereby shaping a recognizable chromatic identity within the city’s urban landscape (Figure 7).

4.2. Urban Building Analysis and Classification in Gros

In his doctoral dissertation La intervención energética en el patrimonio edificado residencial—Análisis del barrio de Gros de Donostia-San Sebastián (2017) (Energy intervention in residential built heritage—Analysis of the Gros neighborhood of Donostia-San Sebastián), Eneko Jokin Uranga Santamaría [52] identifies five principal styles and phases in the twentieth-century residential architecture of the Gros neighborhood.

The first, referred to as the Nineteenth-Century Style (1900–1935), encompasses buildings whose architectural conception and construction techniques maintain a clear continuity with nineteenth-century practices (Figure 8). This architectural language, deeply rooted in San Sebastián since the early Ensanches, drew strongly on French traditions and relied on local materials—most notably sandstone from the Igueldo quarries—which gives these buildings a distinctive character. Although not exclusive to the Gros neighborhood but rather present throughout the city, this style represents a traditional yet relatively advanced mode of construction for its time. Within this framework, various architectural currents are included—such as Neoclassicism, Eclecticism, and Modernism. Structurally, these buildings are organized around perimeter load-bearing walls with an internal timber framework, resulting in façade openings limited by their structural function. From 1900 onward, flat roofs became increasingly common.

During this period, those who could afford it used stone, while those with limited budgets resorted to render, molded cement-mortar ornaments, and mortar moldings. The most widely used color was that of sandstone, as the intention was to simulate that all the less expensive material was in fact stone. This decorative strategy produced an architecture that maintained the level of elegance and decorum demanded by both society and the municipal authorities, while remaining economically accessible.

In the 1920s and 1930s, prefabricated reinforced concrete bay windows also became popular, often incorporating simulated stone block joints. Façade articulation increased, with the multiplication of planes, the incorporation of balconies and projecting bay windows, and the application of a wide variety of artificial stone claddings featuring rustic finishes, rustication, and diverse geometric or rockwork patterns. Ground floors typically retained original stone cladding, whereas cement mortar predominated from the first floor upward.

The “Beaux-Arts” stylistic influence added the gray of limestone to the traditional sandstone color. Various types of marble and lighter-colored stones were also imitated.

Architectural forms associated with workers’ housing, factories, warehouses, and other basic economic activities were resolved through simplified neoclassical compositional formulas devoid of ornamentation. In these cases, there was no molded decoration; façades were finished entirely with render. Within this category, renders ranged from sandstone tones to the white of simple limewash in the most modest buildings.

The second phase, the Rationalist Style (1930–1945), is linked to the widespread introduction of reinforced concrete, which represented a radical transformation in both construction systems and the architectural expression of residential buildings in Gros. This material reduced costs and accelerated construction processes while fostering a profound aesthetic renewal aligned with European modernist currents. Rationalism broke with inherited formal traditions, freed the façade from its structural function, and enabled greater compositional creativity, reducing dependence on stone as the dominant material. The independent structural frame opened new possibilities for design and experimentation, of which significant examples from the 1930s are preserved in the neighborhood.

With regard to color, two new tendencies emerged. First, the white render characteristic of the Modern Movement became prominent, with the most avant-garde buildings featuring white exteriors. Second, the bichromatic schemes associated with Art Deco introduced a new range of pastel tones in renders, along with a diversity of claddings comparable to that of the previous period.

Continuity with nineteenth-century eclecticism can be observed in the regionalist and historicist styles that gained prominence in the 1920s. Party-wall residential buildings were constructed in Neo-Baroque, Neo-Renaissance, or regionalist styles (Figure 9). Once again, molded and painted cement-mortar ornaments assumed a leading role. Façades displayed pediments, moldings, columns, and crestings inspired by major monuments of Spanish architecture from the corresponding historical periods.

The third period corresponds to the Postwar Style (1940–1945), shaped by the aftermath of the Spanish Civil War and the Second World War. The severe shortage of economic resources decisively conditioned architectural production, steering it toward extremely austere solutions. Nevertheless, this period also saw the consolidation and adaptation of techniques derived from Rationalism, incorporating practical improvements that responded to urgent housing needs and construction efficiency.

With regard to color, sandstone-toned render regained prominence, reflecting an aspiration to return to the tradition of local stone (Figure 10). In a context of acute economic crisis, traditional ashlar masonry was simulated using mortar and paint. Coarser textures reappeared, and façades were composed through the alternation of smooth render surfaces and artificial stone cladding.

Brick cladding gained importance, along with terracotta-colored render reminiscent of the “Madrid de los Austrias,” one of the most powerful aesthetic references of the Franco regime. The alternation of “sandstone/terracotta” tones became highly characteristic of architectural production during this period.

The fourth period corresponds to the Style of the Early Developmentalism Phase (1950–1970). With the gradual economic recovery, construction activity increased significantly, particularly in the residential sector. New materials associated with industrial improvement and technological sophistication were introduced, including ceramic mosaic tiles (gresite), glass, and metal joinery (Figure 11). Moreover, as the Franco regime opened to Europe and the United States, new styles, trends, and aesthetic influences emerged.

The load-bearing structure began to express its geometry externally, and molded ornamental elements gradually disappeared. Glazed and more vivid colors gained prominence, while exposed brick increasingly replaced painted cement render. Although traditional chromatic tones did not disappear entirely, they lost their centrality in favor of a more diverse palette.

Finally, the Style of the Second Developmentalism Phase (1965–1980) represents an evolution of the previous stage. In a context of greater economic and technical resources, the use of higher-quality materials became widespread and new construction solutions available on the market were incorporated (Figure 12).

The architectural language maintained the formal direction of Developmentalism but displayed a clear refinement in finishes, along with the introduction of elements such as large glazed openings and projecting bay windows, which enhanced natural lighting and strengthened the relationship between interior spaces and the urban environment. This resulted in noticeable improvements in habitability and comfort.

At the same time, designers intensified experimentation with forms and volumes, giving rise to buildings that, beyond meeting functional requirements, incorporated a more elaborated aesthetic and compositional dimension.

This period also marked the arrival of abstract informalism in architecture, manifested through a heightened sensitivity to the expressive capacity of materials. Façades began to display powerful sculptural volumes reminiscent of Brutalist approaches, expressed in both curved and orthogonal versions, and executed in sandstone, limestone, and other types of stone. Façade planes multiplied, introducing new spatial complexity, with projecting and recessed parapet balconies, as well as continuous vertical, horizontal, or diagonal bands.

A particularly noteworthy case within this volumetric trend is that of brick (Figure 13). After decades of conventional masonry or direct imitation of exposed brick through cladding, new bonding patterns emerged that combined different configurations, restoring a renewed expressive capacity to the material. Greater diversity also became evident in terms of color, size, and texture, with a deliberate effort to enhance the “textile” qualities of brick surfaces and to extract richer visual effects from their material characteristics.

4.3. Determination of the Traditional Colors of the Gros Neighborhood

The characterization of traditional colors in the Gros neighborhood is carried out by considering the full set of buildings within the study area, including both protected and non-protected structures. This comprehensive approach ensures a representative understanding of the chromatic identity of the urban fabric. In the case of transformed buildings, the analysis is based on their original color schemes, prior to the interventions that altered their appearance. This allows for the reconstruction of the historical chromatic context and avoids distortions derived from more recent modifications.

The sample was defined to ensure proportional representation of the five architectural periods identified by Uranga Santamaría [60], as well as variation in protection status and degree of façade transformation. Buildings with intact traditional finishes were examined alongside those recently rehabilitated, allowing comparative assessment of chromatic continuity and disruption patterns.

Following this classification phase, fieldwork was conducted through direct chromatic measurement using a PCE-XXM 30 portable colorimeter. Measurements were taken on representative façade surfaces, avoiding areas affected by shadow, deterioration, or temporary elements, in order to obtain reliable colorimetric data for subsequent analysis (Figure 14).

The methodology involved performing four measurements at different points on the same sample in order to quantify pigmentation variations within a single material unit.

The steps followed for data collection are described below, illustrated through a sandstone sample taken from the façade of the building located at 40 Zabaleta Street. First, a representative sandstone element was selected, and the colorimeter was positioned on the surface to be analyzed, ensuring that the measurement aperture was completely covered in order to prevent stray light interference and guarantee the reproducibility of the reading.

In order to obtain a color code with the highest possible precision, multiple measurements were carried out at different points on the same sample, thereby minimizing errors associated with material heterogeneity. The measurement points were selected to encompass areas with slight apparent variations in texture and tone, with the aim of assessing the intrinsic variability of the stone substrate (Figure 15).

The colorimeter used is equipped with dedicated software that enables comparison among samples and the determination of color differences between them. To this end, a reference sample is defined, and the device’s integrated “PASS/FAIL” function establishes whether the measured values fall within the previously defined parameters. In this system, a red label indicates that the sample deviates excessively from the reference standard, whereas a green label signifies that the sample lies within the established tolerance ranges.

In the case analyzed, Samples 1 and 3 exhibit a high degree of chromatic proximity to the reference sample, while Samples 2 and 4 are not validated by the software, as they exceed the defined color-difference thresholds (

Table 2. Colorimetric measurements obtained from the analyzed sandstone surface, including the reference sample and four test samples with their corresponding CIELAB, RGB, and HSL values.

Sample	Software	Lab Code	RGB Code	HSL Code
Colorimeter Reference Sample		L: 57 a: 5.9 b: 30.3	R: 164 G: 131 B: 69	H: 39.158° S: 41% L: 46%
Sample 1		L: 57.7324 a: 6.3157 b: 31.0834	R: 167 G: 133 B: 69	H: 39.184° S: 42% L: 46%
Sample 2		L: 55.2815 a: 5.2131 b: 29.0046	R: 157 G: 127 B: 68	H: 39.775° S: 40% L: 44%
Sample 3		L: 55.8583 a: 5.3015 b: 29.2385	R: 159 G: 129 B: 69	H: 40° S: 40% L: 45%
Sample 4		L: 59.1991 a: 5.5211 b: 30.4891	R: 159 G: 137 B: 74	H: 39.789° S: 39% L: 48%

). Consequently, the representative color of the analyzed sandstone corresponds to that of the reference sample, characterized by the following parameters: hue (H) = 39.158°, saturation (S) = 41%, and lightness (L) = 46%.

In addition to point-based colorimetric analysis, the study systematically identified the chromatic range of original façade materials that had not undergone significant alteration or differential aging. The objective of this phase was to establish a reference chromatic palette composed exclusively of materials in a stable condition and representative of the building’s original appearance, thereby preventing the introduction of dissonant tones in future interventions.

To this end, construction elements whose chromatic integrity was considered verifiable—such as stone pieces with minimal weathering, unrepaired mortars, and ceramic fragments preserved in situ—were selected and subjected to additional colorimetric measurements following the same protocol described previously. Comparison of the recorded values made it possible to define a homogeneous chromatic range, useful both for characterizing the original material and for establishing chromatic compatibility criteria within the framework of architectural conservation.

In addition, a detailed graphic survey of the façades documented each building’s formal configuration and mapped the chromatic distribution of each construction element (Figure 16). This process included the precise delineation of wall planes, plinths, joinery, balconies, railings, and other architectural components, assigning to each the color obtained through colorimetric measurement. The resulting façade drawings constitute a chromatic cartography of the building ensemble, enabling an integrated visualization of both the compositional structure of the properties and their chromatic performance in relation to the reference palette.

4.4. Identification of Chromatic Alterations in the Gros Neighborhood

The Chromatic Integration Percentage (CIP) methodology was applied to the building ensemble of the Gros neighborhood to compare chromatic coherence across the properties that compose this urban environment. For each building, the analysis identified the colors on the original elevations and in their current condition [9] and assessed their compatibility with the previously defined baseline chromatic palette. These results supported the calculation of a CIP value for each façade, enabling analysis of both the historical chromatic condition [34] and the alterations introduced over time (Figure 17).

4.5. Chromatic Alteration Risk Index of Buildings in the Gros Neighborhood

The proposed methodology was applied to all buildings of the area, with the aim of evaluating the Chromatic Alteration Risk Index (CARI) of their façades and analyzing the spatial distribution of risk within the selected urban ensemble.

For each building, the analysis systematically applied the four factors defined in the methodology: the Material Factor (MF), the Level of Protection Factor (LPF), the Adjacent Altered Buildings Factor (AABF), and the Building Conservation Status Factor (BCSF). Multiple information sources informed each factor, combining documentary analysis with direct field observation.

On-site analysis determined the Material Factor (MF) by identifying each façade’s predominant materials and applying the weighting criteria defined in the methodology. The analysis derived the Level of Protection Factor (LPF) from the City Council of San Sebastián’s Plan Especial de Protección del Patrimonio Urbanístico Construido (PEPPUC), assigning values according to each building’s regulatory level of protection.

Direct on-site observation also informed the Adjacent Altered Buildings Factor (AABF) by accounting for the presence and number of nearby buildings that exhibit evident chromatic alterations. In addition, the analysis established the Building Conservation Status Factor (BCSF) according to the scale defined in the Basque Government’s Technical Building Inspections methodology and refined the rating through fieldwork to reflect the actual condition of the façades under study.

Once the normalized values of the four factors were obtained, the formula defined for calculating the CARI was applied, classifying each building within the established risk categories (very low, low, moderate, high, and very high risk).

Of the 147 buildings analyzed, 43 were found to have already undergone chromatic alteration, while 104 maintained, at the time of the study, a chromatic condition compatible with the reference palette. Among the 43 altered buildings, 19 possess some degree of regulatory protection, highlighting the occurrence of chromatic alterations even in properties subject to heritage regulation.

Among the 104 non-altered buildings, the CARI analysis reveals a significant level of risk: 20 buildings present a very high-risk index, 55 a high risk, 20 a moderate risk, and only 9 a low risk. These results demonstrate a high degree of chromatic vulnerability within the ensemble, even among buildings that have not yet undergone visible alterations (Figure 18).

The cartographic representation of the CARI, through a map identifying risk levels according to the established chromatic thresholds, clearly visualizes the spatial distribution of vulnerability. Observation of this map reveals the existence of chromatic contagion clusters, understood as areas in which the concentration of altered and high-risk buildings generates a multiplier effect on adjacent façades. Based on this spatial interpretation, a specific map identifying contagion clusters was produced, constituting a key tool for prioritizing interventions and for the preventive management of urban color.

5. Results

The application of the proposed methodology in the Gros neighborhood has enabled the quantification of façade chromatic integration (CIP) and, complementarily, the estimation of the Chromatic Alteration Risk Index (CARI) for the entire set of analyzed buildings.

Taken together, the Material Factor (MF), Level of Protection Factor (LPF), Adjacent Alteration Factor (AABF), and Building Conservation State Factor (BCSF) support a comprehensive interpretation of the phenomenon. Chromatic alteration operates as a cumulative, spatially driven process. It intensifies where material vulnerability, the absence of effective chromatic regulation, and proximity to previously altered cases converge.

5.1. Identification of Clusters and Spatial Pattern of Propagation

A chronological cartographic analysis of façade interventions between 2011 and 2024 reveals a non-random pattern of chromatic transformation (Figure 19). Rather than occurring uniformly across the district, alterations emerge in localized clusters and intensify progressively over time. The observed clustering dynamics are consistent with the urban chromatic epidemiology framework proposed in this study, demonstrating proximity-driven transmission patterns comparable to contagion processes.

In the early period (2011–2013), dissonant interventions appear sporadically and remain spatially isolated. However, from 2015 onward, new alterations tend to concentrate around previously modified buildings. By 2018–2019, and especially during the 2021–2024 period, the pattern becomes clearly cumulative, with expanding clusters forming around initial cases and generating identifiable hotspots.

This diachronic mapping demonstrates that chromatic alteration behaves as a path-dependent process: early interventions function as visual precedents that increase the likelihood of subsequent modifications in adjacent buildings. The temporal dimension thus reinforces the spatial logic embedded in the Chromatic Alteration Risk Index (CARI), particularly through the weight assigned to the presence of previously altered neighboring façades (Adjacent Altered Buildings Factor, AABF).

The cartographic representation of the CARI, through a map identifying risk levels according to the established chromatic thresholds, clearly visualizes the spatial distribution of vulnerability. The identification of clusters is based on a systematic visual and spatial interpretation of this cartographic output, focusing on the concentration and proximity of buildings with high CARI values.

In this study, clusters—referred to as “chromatic contagion clusters”—are defined as spatial groupings of buildings exhibiting high levels of chromatic alteration or risk, where adjacency and visual continuity suggest potential interaction effects. The delineation of these areas was carried out through the analysis of spatial contiguity and density of high-risk cases, rather than through the application of formal spatial statistics such as kernel density estimation or autocorrelation indices.

Based on this interpretation, a specific map identifying these clusters was produced, constituting a key tool for prioritizing interventions and for the preventive management of urban color.

The results identify four distinct clusters of chromatic alteration—areas where buildings concentrate dissonant color ranges relative to the traditional palette and where spatial proximity may contribute to the reinforcement of these patterns in their immediate surroundings (Figure 20).

• Cluster 1 (northeast sector of the neighborhood): Segundo Ispizua—Zabaleta—Bermingham

This is the most intense cluster. The first alterations occurred in 2011 and are concentrated in buildings with rendered and painted façades—a typology particularly susceptible to chromatic change due to the technical ease of repainting and the absence of “material anchors” (such as exposed stone) that might constrain color decisions (Figure 21). A significant finding is that there are no heritage-protected buildings in the immediate surroundings, reinforcing the weight of the Level of Protection Factor (LPF)—specifically the absence of regulatory constraints—and, above all, the Adjacent Altered Buildings Factor (AABF), thereby consolidating this cluster as an active propagation core.

• Cluster 2 (San Francisco Street area)

This cluster exhibits a different pattern, generating a more dispersed “stain” with prolonged temporal evolution. Alterations began around 2013, followed by subsequent interventions in 2015, 2018, and 2021, suggesting a sustained and periodically reactivated focus. This cluster tends to extend toward the southern part of the neighborhood. However, along its northern edge, protected buildings function as a barrier or “vaccine,” reducing the continuity of contagion. This finding is particularly significant, as it demonstrates that heritage protection—when effectively implemented and applied to envelopes with greater “material resistance”—can slow the process, producing discontinuities in the risk gradient.

• Cluster 3 (near the coastline): Ramón y Cajal—Colón—Claudio Antón de Luzuriaga

In this case, alterations have occurred in protected buildings which, despite having heritage recognition, feature rendered and painted surfaces. The results indicate that protection alone does not guarantee chromatic conservation if the applicable regulations do not establish specific chromatic criteria. In other words, although the Level of Protection Factor (LPF) is formally reduced (due to the existence of protection), the absence of chromatic prescriptions maintains a high level of practical vulnerability.

• Cluster 4 (southwest sector): Zabaleta—Colón—Nueva—Miracruz

A pattern similar to that of Cluster 3 is observed: protected buildings with rendered and painted surfaces, and regulatory frameworks lacking explicit chromatic determinations. However, an important nuance is detected here: the presence of surrounding protected buildings results in alterations affecting smaller surface areas or manifesting in a more contained manner. This suggests that surrounding protection contributes to limiting the expansion of contagion, even if it does not eliminate it entirely.

Taken together, the four clusters confirm that chromatic alteration does not occur randomly but follows a logic of concentration and propagation: clusters act as initial nodes, and the probability of new alterations increases in adjacent buildings.

5.2. Relationship Between Materiality and Risk: The Role of the Material Factor (MF)

The analysis reveals that façade material is a key determinant of risk. Rendered and painted façades (and, more generally, systems with replaceable or repaintable surface finishes) are associated with higher CARI values. This finding is consistent with the logic of the Material Factor (MF): when the material facilitates the substitution of finishes and the adoption of new aesthetic schemes (e.g., cool tones, whites/greys, or high-contrast compositions), chromatic alteration becomes more likely and easier to implement (Figure 22).

By contrast, stone façades—particularly when combined with heritage protection—demonstrate greater chromatic stability. They not only exhibit lower susceptibility to repainting but also maintain a more direct connection with the traditional palette associated with the neighborhood’s sandy and ochre tones, thereby acting as a material support for the “inherited urban color.”

5.3. Heritage Protection and Its Effective Scope: When Color Is Not Included

One of the most significant findings of this research is the distinction between “formal” heritage protection and “effective” chromatic protection. In Clusters 3 and 4, it is evident that protected buildings have undergone substantial chromatic alterations. The primary cause does not lie in the absence of protection, but rather in the fact that existing regulatory frameworks do not incorporate specific chromatic provisions. This situation creates an operational gap: formal protection covers the building, yet the “color” parameter remains subject to interpretation or individual decision-making (Figure 23).

This result reinforces the central argument of the article: the conservation of the urban landscape in historic centers requires specific instruments for chromatic regulation. Conventional protection mechanisms do not necessarily safeguard color when they prioritize volumetry, composition, or ornamental elements while omitting finishes and chromatic palettes.

5.4. Evidence of the “Contagion Effect”: Weight of the Adjacent Altered Buildings Factor (AABF) and Spatial Gradient

The results provide empirical support for the spatial patterns described in the introduction as a potential “contagion effect”. The variable exhibiting the clearest spatial behavior is the Adjacent Altered Buildings Factor (AABF), which increases the CARI according to the presence of nearby buildings that have already undergone chromatic alteration. In dense urban fabrics such as Gros, where the distance between façades is minimal and collective perception is constructed through visual continuity, proximity to a dissonant case may contribute to a process of normalization: the altered intervention ceases to be perceived as an exception and instead becomes a possible reference for subsequent rehabilitation processes.

However, these spatial patterns should be interpreted with caution. The observed clustering of chromatic alterations does not in itself demonstrate a causal process of contagion or imitation. Alternative explanations may also account for these distributions, including synchronized rehabilitation cycles, the influence of public subsidy programs, shared ownership structures, similar construction typologies, common maintenance timelines affecting groups of buildings, and the widespread use of standardized contemporary materials characterized by homogeneous colors and finishes.

Consistently, a risk gradient can be observed (Figure 24): vulnerability tends to decrease as the distance from clusters or from the nearest altered buildings increases, particularly when “barrier elements” (such as protected buildings or façades with high material stability, e.g., exposed stone) intervene and interrupt visual continuity. These elements may act as spatial buffers, limiting the spread of alterations within the urban fabric.

In this context, the notion of “contagion” should be understood as an interpretative framework that helps describe the spatial correlation and potential diffusion of chromatic changes, rather than as direct evidence of a deterministic or exclusively causal mechanism.

5.5. Implications for Urban Management: Toward Chromatic Regulation in Rehabilitation Processes

The identified patterns provide a basis for deriving practical implications for urban color management. However, these implications should be understood as informed proposals, rather than as deterministic prescriptions derived directly from the data. In this context, the application of the CARI and the identification of spatial clusters suggest potential strategies for intervention, including:

Prioritizing areas where chromatic guidelines or color catalogues could be implemented, particularly in cluster cores and their immediate surroundings.

Defining differentiated approaches according to façade typology, as rendered and painted façades exhibit higher levels of chromatic variability than exposed stone façades.

Reinforcing existing protection frameworks by incorporating color as an explicit regulatory parameter, especially in buildings that, despite being protected, remain materially vulnerable.

The application of the proposed methodology to the Gros neighborhood (San Sebastián) has enabled a detailed characterization of the spatial and typological patterns associated with chromatic alteration. Based on the defined factors—façade material (MF), level of protection (LPF), presence of already altered adjacent buildings (AABF), and conservation status (BCSF)—the Chromatic Alteration Risk Index (CARI) was calculated for each property, integrating its constructive, regulatory, and contextual characteristics.

The results indicate the presence of spatial concentrations of chromatic alteration, understood as clusters of buildings exhibiting similar risk profiles. These clusters correspond to areas where buildings that have undergone significant chromatic change are more densely distributed. The analysis shows that non-protected buildings located in proximity to these areas, particularly those with rendered and painted façades, tend to exhibit higher risk values. In such cases, the combination of material susceptibility, absence of regulatory constraints, and proximity to already altered buildings is associated with increased levels of chromatic alteration risk.

A spatial gradient is also observed: as the distance from altered buildings increases, the risk index tends to decrease. This pattern highlights the relevance of the AABF variable within the model, particularly in dense urban fabrics where visual and spatial proximity between façades is high.

Conversely, highly protected buildings (GR1) tend to exhibit low risk values, even when located near areas with higher concentrations of alteration. This may be related to their lower material susceptibility. However, this effect is less evident in protected buildings with rendered façades, where material vulnerability remains a significant factor.

In this sense, the findings do not establish deterministic relationships, but they do provide a robust analytical basis that can inform urban planning and heritage management strategies. In particular, they highlight the potential value of incorporating chromatic criteria into regulatory frameworks and of adopting differentiated approaches based on façade vulnerability and spatial context.

6. Conclusions

6.1. General Conclusions

The results of the study conducted in the Gros neighborhood (San Sebastián) demonstrate that the current wave of envelope rehabilitation—driven by energy efficiency programs—is generating chromatic and material transformations that extend beyond the individual building and affect the urban landscape as a systemic whole. In the absence of specific criteria regarding color, finishes, and materials in consolidated urban fabrics, interventions tend to introduce standardized solutions (cool color ranges, whites and greys, high contrasts, and patterns inspired by contemporary aesthetics), resulting in a progressive loss of visual continuity and a weakening of the urban identity historically associated with shared palettes. This trend compromises not only aesthetic values but also the cultural sustainability of the built environment, eroding neighborhood legibility and its capacity to function as a repository of collective memory.

A central contribution of this work is the identification of non-random spatial patterns in chromatic alteration. The analysis reveals the presence of transformation clusters that can be interpreted as potential nuclei of propagation, suggesting a spatial correlation in the distribution of alterations. In this sense, the results are consistent with what has been described in the literature as a possible “contagion effect”, whereby the presence of already altered buildings may influence the likelihood that subsequent rehabilitation processes adopt similar chromatic solutions in their immediate surroundings.

In dense urban fabrics with strong perceptual continuity, dissonance may cease to be perceived as an exception and instead become progressively normalized through repetition, contributing to processes of visual homogenization and potential erosion of local character.

This finding is particularly relevant because it shifts the debate from purely subjective evaluations of “good” or “bad” taste toward a preventive risk-management perspective grounded in the identification of spatial patterns.

Within this framework, the proposed methodology—through the Chromatic Integration Percentage (CIP) and the Chromatic Alteration Risk Index (CARI)—provides an operational tool for translating a qualitative phenomenon (chromatic coherence) into comparable indicators applicable at the urban scale. The CIP enables assessment of each façade’s chromatic compatibility with a baseline palette, while the CARI integrates constructive, regulatory, and contextual variables to anticipate the probability of future alteration. The combined use of both indicators offers a dual reading of particular utility for public administration: (1) diagnosing the current state of chromatic integration within the building fabric and (2) identifying, prioritizing, and hierarchizing areas where regulatory control and guidance are most urgently required.

The study also demonstrates that conventional heritage protection, when it does not incorporate explicit chromatic determinations, is insufficient to guarantee the conservation of the urban landscape. A distinction emerges between “formal” and “effective” protection: while the existence of protective designations may reduce vulnerability in certain aspects (volumetry, composition), it does not prevent chromatic alterations if color is not regulated [29]. This finding suggests the need to update protection and planning instruments by incorporating color as a normative parameter, particularly in buildings whose materiality (rendered and painted surfaces) facilitates rapid and visually impactful transformations.

From a public policy perspective, the implications are direct. First, historic neighborhoods should not treat color management as a set of general recommendations; binding chromatic guidelines—or color and material catalogues that define acceptable ranges and verification procedures—should support it [9]. Second, regulatory intervention should be spatially targeted, acting more intensively in the cores and edges of identified clusters—where contagion is more likely—and applying differentiated strategies according to façade typology: rendered and painted surfaces require stricter chromatic control than exposed stone façades due to their greater susceptibility to alteration. Third, the preventive approach enabled by the CARI allows for the optimization of public resources: rather than intervening once landscape degradation has become widespread, selective action can be directed toward buildings with high or very high risk, reinforcing chromatic coherence before dissonance spreads.

In summary, this study concludes that energy retrofitting in consolidated urban environments requires a more precise framework of aesthetic and heritage governance capable of reconciling climate objectives with the preservation of visual identity. The proposed methodology offers a replicable and transferable basis for supporting planning decisions, designing chromatic regulatory tools, and guiding future interventions. Its application enables a shift from reactive to preventive management of the historic urban landscape, avoiding the irreversible erosion of coherence and character resulting from the cumulative effect of individual decisions.

Finally, the results confirm the suitability of adopting an urban chromatic epidemiology approach to analyze landscape transformation dynamics in consolidated fabrics. The identification of clusters, detection of spatial propagation patterns, and modeling of risk make it possible to interpret chromatic alterations not as isolated episodes but as cumulative processes subject to dynamics of exposure, susceptibility, and contextual transmission. This framework provides a conceptual and operational basis particularly useful for anticipating change scenarios and guiding preventive urban management strategies.

6.2. Limitations of the Study and Future Research Directions

The results obtained in this research highlight the usefulness of the proposed methodology for identifying, measuring, and anticipating processes of chromatic alteration in consolidated urban fabrics. Nevertheless, as with any study applied to complex urban contexts, its scope must be understood in relation to the methodological decisions adopted, which define both its strengths and its potential for future development.

First, the analysis focuses on a single case study—the Gros neighborhood in San Sebastián—selected for its typological diversity, its historically coherent chromatic identity, and the recent intensity of façade interventions. This focus has enabled a detailed and contextualized reading of the phenomenon, appropriate for the initial validation of the methodology. Although the results do not automatically generalize to other contexts, the method’s structure was designed from the outset to be replicable, allowing its application to other historic neighborhoods and urban extensions with different morphological, chromatic, and regulatory characteristics.

Regarding the determination of the reference chromatic palette, it was based on colorimetric measurements carried out on materials considered representative of the original or stable condition of façades. It is acknowledged that factors such as weathering, environmental pollution, or cumulative maintenance interventions over time may introduce variations in the measured values. However, the methodology incorporates specific procedures—such as multiple measurements, the selection of homogeneous surfaces, and software-based validation—that reduce these variations and allow the definition of chromatic ranges sufficiently robust for use as compatibility criteria. In this sense, rather than reconstructing an exact and immutable “original color,” the method seeks to identify a coherent chromatic framework reflecting the material and perceptual identity of the context [29].

Similarly, the Chromatic Integration Percentage (CIP) and the Chromatic Alteration Risk Index (CARI) are based on a deliberate simplification of a complex urban reality. The discretization of variables and the assignment of homogeneous weights to the analyzed factors were adopted in order to generate clear, operational, and easily interpretable indicators for municipal technicians and rehabilitation professionals. Far from constituting a weakness, this decision strengthens the practical applicability of the method. Nevertheless, future research could refine the calibration of these indices by exploring differentiated weightings or incorporating statistical analyses to fine-tune their performance across diverse urban contexts.

Moreover, the study has focused specifically on the chromatic parameter, understood as one of the principal vectors of perceptual alteration in the urban landscape. Other relevant aspects—such as texture, material reflectance, interaction with natural and artificial lighting, or the relationship between color and architectural form—have not been explicitly integrated into the proposed indices. This delimitation responds to the need to clearly define the object of study and ensure methodological clarity, but it also opens a complementary field of research that could enrich the perceptual evaluation of the built environment.

Based on these considerations, several future research directions can be identified to consolidate and expand the methodology. First, its application in other historic neighborhoods and urban extensions would allow testing its robustness, adjusting reference chromatic ranges, and verifying the reproducibility of the chromatic contagion phenomenon in diverse contexts. Second, the validation of the CIP and CARI indicators through expert review processes and citizen perception studies would strengthen their empirical foundation, linking quantitative results with levels of visual comfort, social acceptance, and place attachment.

Furthermore, future research could incorporate a more developed temporal dimension, through predictive models capable of simulating the chromatic evolution of urban fabrics under different regulatory and intervention scenarios. Such an approach would enable prospective evaluation of rehabilitation policies and reinforce the preventive character of the proposed methodology.

Finally, the integration of these indicators into digital urban management tools—such as geographic information systems, permitting platforms, or interactive chromatic catalogues [9]—represents a particularly promising line of development. Their implementation would facilitate the transfer of research findings into administrative practice, contributing to more informed urban color management and supporting energy retrofitting processes compatible with the preservation of the historic urban landscape and the cultural sustainability of cities.