Glossary · Accessibility term

Color blindness

Also called: Color vision deficiency

Color blindness is a reduced ability to tell certain colors apart, and the figure everybody quotes, roughly 1 in 12 men and 1 in 200 women, is for red-green deficiency among people of Northern European descent. Prevalence is lower in other populations, so read it as an order of magnitude rather than a headcount for your audience. The common forms make red and green hard to separate, which is why red error states, green success states and red-and-green chart series are the classic failures. It reaches further than the name suggests, though. Red-green deficiency also confuses blue with purple, because purple has red in it. A second and rarer form runs on blue and yellow instead, so switching your chart to those is not the safe answer people assume. Seeing no color at all is rare. The rule that follows from all of it fits on one line. Never make color the only way to tell something.

In practice

It is not a contrast problem, and that distinction is the most useful thing on this page. Text can measure 4.5:1 and lose all its meaning if the meaning was carried by hue. A required field marked only by turning its label red. A status shown only as a colored dot. A legend that maps each series to a color and gives you nothing else. The asterisk beside a required field is the one people report by mistake, because an asterisk is a shape and a shape is not a color, so what it needs is an explanation rather than a second cue. Use of Color is a separate rule from contrast, it sits at Level A rather than AA, and passing the contrast rule tells you nothing whatever about it.

Its scope runs past information, too. The rule covers indicating an action, prompting a response, and distinguishing one visual element from another. So a link that stands out from the text around it only by being blue fails it, even though no information is being conveyed. Underline it, or give it something other than hue to be recognized by.

Two things widen the audience well past the headline number. A lot of people do not know they have it, since the symptoms are often mild enough to go unnoticed and people adjust without ever being tested. And it is not only inherited. Retinal injury, optic nerve damage, diabetes, multiple sclerosis, some medicines and cataracts with age all cause it, which means this group grows along with the average age of your customers.

Why it matters

Somebody who cannot tell your red error state from your gray body text will not report a color problem. They will report that the form is broken, or they will leave without reporting anything at all, because nothing on screen told them a field needed fixing. That is the real cost of using hue as the only signal, and it is why this failure is so badly under-reported. Fixing it is a matter of adding a second cue rather than changing the palette. An icon, a word, an underline, a pattern in the chart, a label on the line.

The ten-second test, and its limit

View the page in grayscale. Anything that turns ambiguous was relying on color, and it is the fastest sweep there is. Treat it as a sweep rather than a verdict, because a pair can separate in grayscale and still fail for one particular kind of deficiency, red text on a near-black background being the known case. A chart legend can fail twice over, once for mapping meaning to hue and again where the adjacent series colors are what you need to tell the series apart and do not clear 3:1.

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