09/05/2018
Any information conveyed by color MUST be accompanied by a programmatically-discernible text alternative.
When color is used as the only means to convey information, people who are colorblind, have low vision or are blind will be unable to take advantage of or make sense of the information. For example, a person who cannot see will not know which to choose when instructed to "advance to next screen using the red arrow", and a person with low vision or color blindness will not be able to use a color-coded map of bus routes if color is the only way the routes are differentiated.
Text is the foundation upon which all other accessible content is built. Everything must exist in a text format — either as normal text or as alt text — or else screen readers won't be able to read it. If you color-code anything on a web page, you'll need to convey the meaning behind the colors in a text format.