Accessible color and contrast

Basics

  1. Never use sensory cues alone to visually convey important information.
  2. Make sure colors have sufficient contrast.

In-depth

Many people have difficulty distinguishing certain colors or colors with low contrast. Users with low vision, color blindness, monochrome screens, screen rendering problems, light interference, and other issues rely on secondary, non-color cues. RIchard Morton of gov.uk gives a revealing example:

Ishihara color test

If you can see the number 74 in the main image above, you may have normal colour vision...and if you can see 21 like me, you may have a red-green colour vision deficiency.

As many as 1 in 8 men and 1 in 200 women have the common form of red-green color blindness (NEI).

Don't use color alone to convey important information

Add secondary cues

Never use color (or any other sensory cues [position, shape, sound, etc.]) alone to visually convey important information. If color transmits something meaningful on a page (such as a link, required field or active state), provide a secondary method to communicate the information. Add an underline, bold, patterns, icons, text additions, or other non-color visual cues.

Color should not be the sole visual cue. Use a secondary cue for users who are color bind.
What if I can't see the color red very well? Underlining links helps users detect linked text.

Add clear directions and labels

Instructions should not rely solely on color. For instance, "red button" would be confusing to users who are blind. If you use such sensory indicators, also include a label in the directions ("red submit button") and ensure the element referred to is properly labeled.

Confusing for blind or colorblind users Add labels to directions to help users.
Click on the green arrow. Click on the green "Next" arrow in the lower right.

Make sure colors have sufficient contrast.

For people with color blindness or other vision problems, high contrast makes text much more legible. Make sure colors on the page and in images, graphs and charts have a strong contrast. Try to achieve these visual color contrast ratios:

  • Between text and background: 4.5:1
  • Between link and non-link text: 3:1
Insufficient contrast Sufficient contrast
Low-contrast font colors make it extremely
difficult for users with low or color vision
deficiencies to read your content.		 Low-contrast font colors make it extremely
difficult for users with low or color vision
deficiencies to read your content.

How to test

Manually evaluate several pages (the homepage, a representative internal page and any pages that contain unique elements). If color visually conveys important information, ensure that an alternative method of conveying that information is present, including secondary cues and textual directions or labels.

A quick way to evaluate contrast is to take a screenshot of a page and change it to black and white. If elements are difficult to distinguish, increase the contrast ratio.

Contrast checkers

Relevant W3C WAI documents