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.
In plain language
If color is used to communicate something — like showing an error, marking a required field, or distinguishing a link from regular text — there must also be another visual cue (text, an icon, an underline, a pattern) that doesn't rely on color alone.
Why it matters
People with color blindness or low vision may not be able to tell colors apart, and anyone viewing a screen in bright sunlight or on a grayscale display loses color information entirely. If color is the only signal, these users miss the information completely.
In practice
Fails this criterion
A form marks invalid fields only by turning the border red. A user with red-green color blindness can't tell which fields have errors.
Meets this criterion
The same form turns the border red and adds an error icon and a text message ("This field is required"). Anyone can identify the error, regardless of how they perceive color.
Test it yourself
Take a screenshot of the page and convert it to grayscale (most image editors have a "desaturate" filter). If you can no longer tell which elements are links, which fields have errors, or which option is selected, this criterion is failing.