Accessible color and contrast

• Don't use sensory cues alone to visually convey important information.
• Make sure colors have sufficient contrast (see also Complex images)

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:

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. For example, one of the most common methods of indicating a link is to underline it.

 

In addition to underlining text, you can also use bold-face, patterns, icons, text descriptions, etc., to add secondary non-color visual cues.

Add clear directions and labels

Instructions should not rely solely on color. For instance, referring to the "red button" is confusing to users who are blind. If you use sensory indicators, also include a text identifier in the directions and as a label on the element.

Make sure colors have sufficient contrast.

For people with color blindness or other vision problems, high-contrast text on the page and in images makes your content much more legible. Try to achieve at least these visual color contrast ratios:

• Regular text must have at least a 4.5:1 contrast ratio between text and background.
• Links must have at least a 3:1 contrast ratio between link and non-link text.

 

For color contrast on complex images like infographics, charts and graphs, see the article on  Complex images.

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

• WebAIM's color contrast checker
• Color Contrast Analyser (Windows and Mac)
• Color Contrast Analyzer (Chrome)
• WCAG Contrast Checker (Firefox)

Relevant W3C WAI documents

• WCAG 2.1 Guideline 1.3.3 Sensory Characteristics: Instructions provided for understanding and operating content do not rely solely on sensory characteristics of components such as shape, size, visual location, orientation, or sound.
• How to Meet WCAG 2.1 Guideline 1.3.3

• WCAG 2.1 Guideline 1.4.1 Use of Color: Color is not used as the only visual means of conveying information, indicating an action, prompting a response, or distinguishing a visual element.
• How to Meet WCAG 2.1 Guideline 1.4.1

• WCAG 2.1 Guideline 1.4.3 Contrast (Minimum): The visual presentation of text and images of text has a contrast ratio of at least 4.5:1.
• How to Meet WCAG 2.1 Guideline 1.4.3