REVIEW 5 major objections 6 minor 1 cited by
Color in Visual-Language Models: CLIP deficiencies
T0 review · 5 major / 6 minor · reviewed 2026-08-08 · deepseek-v4-flash
Pith's one-line read CLIP understands chromatic colors but treats white, gray, and black as non-colors and prefers reading color words over perceiving font colors, a deficiency traceable to text-selective neurons.
desk verdict The paper's behavioral evidence for CLIP's achromatic bias and text-priority is solid and worth engaging with, while the neuron-level mechanistic claims are more speculative and need sharper quantification. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The evaluation machinery is a zero-shot color-naming task: for each image, a set of color-label text prompts is ranked by cosine similarity between the CLIP text and image embeddings, and the highest-scoring label is the prediction. This turns every experiment into a direct comparison of how the text encoder and image encoder align. For the internal analysis, the paper introduces a Color-Label Selectivity Index, $f_c(n_{i,L}) = \frac{\sum_{j=1}^{N_c} w_{j,i,L}}{\sum_{l=1}^{N} w_{l,i,L}}$, which measures how often a specific color label appears among the top-activating images for a neuron. This index is used to classify neurons into five types—Color, Any Word, Color Word, Color Multimodal, and Not activated—and to map their distribution across layers.
What would settle it
Use the same colored-text images but present the answer options as color swatches (patches of color) instead of color-name words, and measure accuracy on achromatic font colors and font-color identification. If CLIP still fails on achromatic colors and still returns the written color name, the deficiency is visual; if accuracy rises sharply, the reported failures are an artifact of prompting with color names.
Extended reading notes
Core claim
On the paper's own terms, the central discovery is that CLIP's color ability is split: it correctly links chromatic color labels to visual stimuli, but it does not treat achromatic stimuli as belonging to the color concept. In the shape experiments, when the asked object is achromatic and the background chromatic, correct object-color prediction falls to 0.19%; when the background is achromatic and the object chromatic, correct background-color prediction falls to 5%. In the Stroop experiments, CLIP's font-color accuracy is 16.7% on a white background and 2.35% on a colored background, while the written color name is chosen 81.1% and 59.5% of the time respectively. The neuron analysis then shows why: deep layers are dominated by text-selective neurons, and the color multi-modal neurons that do exist are few and appear in shallow layers.
Load-bearing premise
The results assume that picking the top-scoring color-label prompt by cosine similarity faithfully measures what CLIP perceives as the image's color, so a bias in how the prompt text is phrased could masquerade as a visual deficiency.
Editorial extensions
If this is right
- CLIP-based zero-shot color classifiers will systematically mislabel white, gray, and black whenever a chromatic color is in view, regardless of which image region the prompt asks about.
- Any image that contains a color word will be labeled by reading rather than by perceived color, so downstream tasks such as font-color extraction or color-aware captioning will fail on such images.
- The reading bias grows with visual distractions: adding a colored background drops font-color accuracy from 16.7% to 2.35%.
- Since text-selective neurons concentrate in deep layers while color multi-modal neurons are few and shallow, the deficiency is structural rather than a simple threshold artifact, and will not be cured by prompt rewording alone.
Reading between the lines
- This suggests a testable extension: if the same Stroop images are evaluated with color swatches instead of color-name prompts as answer options, and achromatic accuracy stays near zero, the failure is in the visual encoder; if accuracy jumps, the reported metric was partly a text artifact.
- The shallow-layer color multi-modal neurons suggest that low-level attribute concepts are encoded earlier in CLIP than previously assumed; probing these neurons might predict a model's color robustness before full evaluation.
- Models finetuned from CLIP for image generation or editing may inherit the achromatic deficiency, producing black, gray, or white objects only with difficulty; this could be checked by prompting such models for achromatic objects in colorful scenes.
- The authors' progressive-training hypothesis could be tested directly by re-training a small CLIP variant on an ordered curriculum and measuring achromatic Stroop accuracy before and after.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This paper presents a set of experiments on synthetic images and Stroop-style stimuli to evaluate CLIP's ability to label colors. The authors report that CLIP correctly labels chromatic colors but fails to assign achromatic labels (white, gray, black) when a chromatic color is present in the image, and that CLIP strongly prefers the written color word over the actual font color in Stroop tasks. To explain these behaviors, the authors introduce a Color-Label Selectivity Index and analyze neuron activations across CLIP's visual encoder, proposing a taxonomy of neuron types and reporting the presence of color multimodal neurons in shallow layers. The main contributions are empirical: a systematic behavioral characterization of CLIP's color biases and a descriptive neuron-level analysis.
Significance. If confirmed, the behavioral findings are valuable for the VLM community, as they highlight systematic color deficiencies in a widely used model. The use of large synthetic datasets with multiple prompt templates is a methodological strength, and the Stroop experiments provide a compelling demonstration of text-over-vision bias. The neuron analysis is exploratory and less central; its contribution would be strengthened by more rigorous definitions and quantitative validation. The paper does not provide code or model specifications, which limits its immediate reproducibility.
major comments (5)
- [Section 2, Experiments 1-2 (Table 2)] The central claim that "CLIP does not attribute the color word to achromatic parts of the image in presence of a Chromatic color" is inferred exclusively from the ranking of 11 color-name text prompts by cosine similarity to the full image embedding. This operationalization conflates the image encoder's representation, the text encoder's embedding of color words, and the contrastive alignment. The paper varies the prompt template (Experiment 3) but never varies the output channel; all answers are text-label rankings. A visual-side control—for example, a linear probe trained on the image encoder's features to predict the actual object/background color, or a forced-choice task with achromatic labels in a matched context—is needed to determine whether the achromatic failure reflects a visual encoding deficiency or a text-encoder prior that achromatic words are less plausible color labels. Without such a control, the causal interpretation of the achromatic deficiency (and the hypothesis in the Conclusions about achromatic backgrounds in datasets) is underdetermined.
- [Section 4, "Activation Analysis" and Figure 5] The neuron taxonomy is defined using unquantified thresholds. The criteria "high Color-Label Selectivity Index", "high activation for any word", "does not reach 50% of the maximum activation", and the conjunction required for "Color Multimodal" are not specified numerically (e.g., percentile cutoffs, minimum activation ratios, or how the three color-label conditions are combined). The reported distribution of neuron types and the finding of shallow-layer color multimodal neurons cannot be reproduced or evaluated without these thresholds.
- [Throughout (all experiments)] The paper does not identify the exact CLIP model variant or implementation used for any experiment. The Introduction says the image encoder is "Resnet-like" and the neuron analysis refers to "blocks of convolutional layers", but the model name (e.g., RN50, RN101, ViT-B/32), the pretraining dataset, and the checkpoint or codebase are not given. All experimental results, and especially the neuron-level analysis, depend on this specificity.
- [Abstract and Section 3 (Experiment 5)] The word "significant" is used without supporting statistics. The paper reports aggregate percentages over 500 samples per condition but provides no error bars, confidence intervals, or significance tests. This is particularly problematic for the claim that achromatic backgrounds have "significant lower errors" in the Stroop test, where the effect appears to be smaller than the main text-over-vision bias. At minimum, the authors should report per-condition variability and avoid "significant" without a test.
- [Section 4, Equation (1)] The definition of the Color-Label Selectivity Index is unclear. The notation uses N_c as a count of images but then sums activation values w_{j,i,L} over j, and the denominator sums over N_l w_{l,i,L}. The text should clarify whether f_c is a count ratio or an activation-weighted frequency, and should specify the set of images ("N cropped top scoring images") over which the index is computed (e.g., which dataset, how crops are selected, how many). This is necessary to interpret the subsequent neuron classification.
minor comments (6)
- [Section 2, Experiment 2] The text says "the performance decreases to 0.19%" when the object is achromatic, but Table 2 shows 0.15% for the corresponding cell; please align the text with the table.
- [Section 3, Experiment 3] The text says "2.23%" for the font-color percentage of the selected prompt, but Table 3 shows 2.35%; please correct.
- [Abstract] The abstract uses "VML" while the rest of the paper uses "VLM"; please standardize.
- [Introduction] Reference [5] contains "Akash Ghosh1 et al." with a stray superscript; please clean up the citation.
- [General] Please provide a reproducibility statement or release the code and data; the paper's value would be greatly increased by including the synthetic dataset generation code and the neuron analysis scripts.
- [Section 4, neuron categories] In the description of the neuron categories, the paper should explicitly define what "high activation" means in terms of the activation distribution (e.g., relative to the top-scoring ImageNet patches).
Circularity Check
No significant circularity: the behavioral results are external evaluations on synthetic data, the neuron taxonomy is descriptive, and self-citations are methodological.
full rationale
The paper's headline deficiencies (achromatic colors rarely selected as labels; text over font color in Stroop tests) are direct measurements on synthetic datasets in Sections 2 and 3. No parameter is fitted to the reported outcome and no equation forces the result by definition: the cosine-similarity ranking over 11 color prompts is an operationalization whose validity can be debated, but it is not a circular reduction. The neuron-level analysis in Section 4 is a descriptive taxonomy: the Color-Label Selectivity Index (Eq. 1) measures activation frequencies on the Stroop dataset and the neuron categories are defined by that index, so the taxonomy is not an independent prediction of the behavioral findings, nor is it used as the derivation of those findings. Self-citations [11,12] provide the base selectivity-index methodology and a comparison distribution; they are not load-bearing verifications of the central claims. Since the central behavioral results are self-contained against external benchmark-like synthetic datasets and the internal neuron analysis is explicitly descriptive, no exhibited circular step meets the quoted-reduction standard.
Assumptions & free parameters
free parameters (4)
- Fixed RGB values for the 11 basic color labels =
not stated in the paper
- Top-100 patch count for the color selectivity index =
100
- Activation cutoff for the 'Not activated' neuron category =
50% of ImageNet maximum
- Color-Label selectivity 'high' threshold =
not specified
assumptions (4)
- domain assumption Cosine similarity between CLIP image and text embeddings is a valid proxy for color understanding
- domain assumption Flat synthetic renders with homogeneous backgrounds and basic shapes represent color perception tasks
- domain assumption The Berlin and Kay 11 basic color terms are the correct closed label set
- domain assumption Top-scoring image patches reveal a neuron's semantic selectivity
invented entities (1)
-
Neuron taxonomy of color-label selective types
Cite this review
Pith. "Pith review of Color in Visual-Language Models: CLIP deficiencies." pith.science (2026). https://pith.science/paper/XQA3BWIM
@misc{pith2026250204470,
author = {Pith},
title = {Pith review of: Color in Visual-Language Models: CLIP deficiencies},
year = {2026},
howpublished = {\url{https://pith.science/paper/XQA3BWIM}},
note = {Machine review of arXiv:2502.04470}
}
read the original abstract
This work explores how color is encoded in CLIP (Contrastive Language-Image Pre-training) which is currently the most influential VML (Visual Language model) in Artificial Intelligence. After performing different experiments on synthetic datasets created for this task, we conclude that CLIP is able to attribute correct color labels to colored visual stimulus, but, we come across two main deficiencies: (a) a clear bias on achromatic stimuli that are poorly related to the color concept, thus white, gray and black are rarely assigned as color labels; and (b) the tendency to prioritize text over other visual information. Here we prove it is highly significant in color labelling through an exhaustive Stroop-effect test. With the aim to find the causes of these color deficiencies, we analyse the internal representation at the neuron level. We conclude that CLIP presents an important amount of neurons selective to text, specially in deepest layers of the network, and a smaller amount of multi-modal color neurons which could be the key of understanding the concept of color properly. Our investigation underscores the necessity of refining color representation mechanisms in neural networks to foster a more comprehensive comprehension of colors as humans understand them, thereby advancing the efficacy and versatility of multimodal models like CLIP in real-world scenarios.
Figures
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Forward citations
Cited by 1 Pith paper
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Language-based Color ISP Tuning
Language-described color styles can be applied to photos by optimizing a small camera color matrix with gradient descent against a vision-language model's similarity score.
Reference graph
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