Some time ago, while pondering whether web accessibility posed limitations on design, the thought occurred to me that there are presumably some colors which simply cannot be used for text or text backgrounds in any site.
WCAG 1 does not, in fact, provide any specific guidelines concerning color contrast. The formulas commonly used to judge this were specified in Techniques For Accessibility Evaluation And Repair Tools, published in 2000. The document is intended to help authors conform to WCAG, but is not actually part of the WCAG document.
The nature of the Web Content Accessibility Guidelines (WCAG) specifications of color contrast fairly well ensures that some colors in the middle range of the spectrum (in hexadecimal, generally between #666666 and #999999) simply won’t be compatible with any other color.
My first thought on this point was to create a chart of colors which simply couldn’t be used in these contexts. I decided against this, on the grounds that it didn’t really seem all that valuable to me. But the thought of viewing color contrast problems in a different way than most color contrast checkers stuck with me.
It seems that most color contrast checking tools work in one of two ways: they either take a webpage and check the contrast factors between text and background on that page, or they allow you to enter a pair of colors and find out how they mesh up.
What I’ve done instead is set up a color contrast checker which only requires you to enter one color, then displays a selection of possible color combinations using that color. It’s pretty straightforward: you can choose to view results ordered according to WCAG 1′s color brightness and color difference tests or according to WCAG 2′s contrast ratio algorithm. Either way, all three factors are displayed, providing a good sense for how the two systems differ.
The results can be quite interesting. Compare the results for
#888888 under WCAG 1 and under WCAG 2, for example:
Altogether, I’m hoping that this tool provides an interesting way to approach color contrast issues and to view the differences between WCAG versions 1 and 2.
- Explanation of luminosity contrast ratio algorithm (Gez Lemon)
- WCAG 2, definition of relative luminance
- WCAG 2, definition of contrast ratio
- WCAG 2, understanding Guideline 1.4
- List of color contrast checking tools (Roger Johansson)
Color Comparison Formulas
Color brightness formula: (must be greater than 125)
((Red value X 299) + (Green value X 587) + (Blue value X 114)) / 1000
Color difference formula: (must be greater than 500)
(maximum (Red value 1, Red value 2) - minimum (Red value 1, Red value 2)) + (maximum (Green value 1, Green value 2) - minimum (Green value 1, Green value 2)) + (maximum (Blue value 1, Blue value 2) - minimum (Blue value 1, Blue value 2))
Contrast Ratio with Relative Luminance formulas: (greater than 3:1 for print sizes over 14pt Bold equivalence or 18pt normal, greater than 4.5:1 for smaller text)
(L1 + 0.05) / (L2 + 0.05) where
L1 = 0.2126 * R1 + 0.7152 * G1 + 0.0722 * B1 on the RGB values of the lighter color and
L2 = 0.2126 * R2 + 0.7152 * G2 + 0.0722 * B2 on the RGB values of the darker color.