Ensure that text and graphics are understandable when viewed without color.
Color should not be the primary means of portraying information on a website. Some people who can’t differentiate between certain colors and users with devices that have non-color or non-visual displays won’t be able to receive certain information. If colors for foreground and background colors are too close to the same hue, may not provide sufficient contrast when viewed using a monochrome display. Or by people with different types of color problems.
Checkpoints:
2.1 - Ensure that all information conveyed with color is also available without color, for example from context or markup.
2.2 - Ensure that foreground and background color combinations provide sufficient contrast when viewed by someone having color deficits or when viewed on a black and white screen.
This was a really short guideline but it is an important one for user with portraying colors correctly with their displays or problems they may have with their eyes. So it will be important to make sure that a website can get across its meaning and purpose without the sole use of color. For more information follow this link.