I am checking a software against the software ergonomic requirements of WCAG 2.1 Level AA. On part is the colour contrast, which must be for graphics 3.0:1. In the following graphic the orange part is for users without any disabilities very good visible, however the tool "Color Contrast Analyser" show only a contrast to the white background of 2.0:1.

So the graphic does not meet WCAG 2.1 Level AA. But what is the reason? Again: the orange part of the graphic is very good visible. Might it be an inaccuracy of algorithm for calculating colour contrast? Or is it just my personal sensation?

Any other ideas?

graphic with red, orange and green skala. medium value is 38.

2 Answers 2


Accessibility is mainly about inclusion. Think about font size for example. A kid might argue that the text doesn't need to be bigger because it can still read/see the tiniest letters. An older person looking at the same text might complain that they'd need a magnifying glass in order to read it.

In the sense of accessibility and inclusion, you should try and design with people in mind, that may have certain difficulties in seeing, hearing etc. So even if it's completely clear and visible to you, keep in mind, that it might not be that clearly visible to everyone.

Another thing to keep in mind, is that there might be some monitors/displays, that don't handle contrast very well - as it is tackled in this answer: https://ux.stackexchange.com/a/20284/127933

That being said, in your case the contrast is not that big of a problem (though it's still not ideal), since that orange segment is just one part of an otherwise perfectly visible component. Therefore, it shouldn't be as easily overlooked as a singular poorly-contrasted object on its own. Gestalt principles, especially the law of closure is working in your favor here.


Short Answer

There's nothing wrong with that graphic at all, (except you can't really see the pointer over that green), but nothing wrong with that orange as shown.

There is however, something wrong with the contrast math being used to calculate the ratio.

Longer Answer

Early in 2019 I became involved with the W3C due to the problems I found in their approach to contrast. This work resulted in the APCA as a new actually accessible method.

Contrast is not primarily about color, it is first about the spatial qualities, meaning size, thickness, font weight. The human vision system detects contrast based more on spatial frequency than an arbitrary luminance ratio.

Also, colors with a lot of red in them will appear darker to color vision deficient, improving their contrast.

Standard vision is the upper left and then various color vision deficiencies

Per APCA that is Lc 38.6, such a contrast is too low for most text, but text is at a higher spatial frequency—smaller, thinner—so it needs more contrast. But his graphic is big and fat, so very low in terms of the spatial frequency.

The difference between a 6px thick line and a 1px thick line is more than two orders of magnitude in terms of contrast sensitivity.

The arbitrary 3:1 ratio preached in some guidelines has no real empirical evidence behind it for this use case.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.