Re: [csswg-drafts] [css-color-6] `contrast-color()` MVP in Level 5 (#9166)

I strongly believe that, following the science, the MVP should be restricted to achromatic colors (black or white, in the limiting case). Here is why:

# Lightness and Chroma/Hue: effect on legibility and reading performance

Studies (Legge 1990) of both people with normal color vision, and people with low vision, have shown that **sufficient luminance contrast** is the primary factor affecting reading speed, and that adding chromatic contrast **does not** further increase reading speed. This is true for both those with low vision and those with normal vision:

> We found no advantages of color contrast for low-vision reading. For text composed of 6° characters, all low-vision subjects read better with luminance contrast than with color contrast.

Beretta summarized the state of text readability research
(as of 2009) as follows:

1. Luminance contrast is the dominant factor to address readability
2. The luminance polarity affects readability (negative polarity is more difficult)
3. The luminance of the background affects readability (lighter backgrounds are preferable)
4. An additional color contrast does not facilitate reading when a sufficient luminance contrast exists
5. On achromatic background, wavelengths at the extremes of the spectrum are more difficult for text.

Indeed, Zuffi _et. al._ (2009) found that for more muted color combinations, the presence of chromatic contrast (measured as Δab) _impaired_ reading performance, compared to luminance contrast (measured as ΔL) alone:

> By using regression analysis we verified that in the first case (“Low Chromatic” group) there is a
significant (p-value = 0.003) **inverse** relationship between reading performance and chromatic contrast; execution times increase as chromatic contrast increases, indicating a **penalizing effect** of this parameter. In the second case (“High Chromatic” group), on the contrary, the relationship is not significant, and this is consistent with the conclusions of other studies, which found that the chromatic contrast does not improve reading performance when a sufficient luminance contrast is present.

See their [Figure 3](https://www.semanticscholar.org/paper/Understanding-the-readability-of-colored-text-by-on-Zuffi-Brambilla/d1b45368f98e02362b6d2a38da96f210a199ccab/figure/5)

## References

- Giordano Beretta (2009) _Text color polarity_ [blog](http://www.mostlycolor.ch/2009/05/text-color-polarity.html)

- G E Legge, G S Rubin (1986) __Psychophysics of reading. IV. Wavelength effects in normal and low vision _  J Opt Soc Am A, **3(1)**: pp 40-51. [abstract](https://pubmed.ncbi.nlm.nih.gov/3950791/)

- Gordon E. Legge, David H. Parish, Andrew Luebker, and Lee H. Wurm (1990)  _Psychophysics of reading. XI. Comparing color contrast and luminance contrast_. Journal of the Optical Society of America vol **7** issue 10 pp. 2002-2010 [https://doi.org/10.1364/JOSAA.7.002002](https://doi.org/10.1364/JOSAA.7.002002)

- Silvia Zuffi, Carla Brambilla, Giordano Beretta, Paolo Scala (2009)
_Understanding the readability of colored text by crowd-sourcing on the Web_
[citation](https://www.semanticscholar.org/paper/Understanding-the-readability-of-colored-text-by-on-Zuffi-Brambilla/d1b45368f98e02362b6d2a38da96f210a199ccab) | [fulltext PDF](http://www.hpl.hp.com/techreports/2009/HPL-2009-182.pdf)

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Received on Tuesday, 29 August 2023 16:49:54 UTC