Color and Graphics
People with color blindness often have difficulty distinguishing certain colors, such as red and green or blue and yellow. When color combinations (such as red text on a green background) are of insufficient contrast, the words can actually disappear to a person with colorblindness or other forms of low vision (cataracts, macular degeneration, etc.).
It is important to select strong, contrasting color combinations and directly labeling things that are otherwise distinguished only by color on a page (think charts and graphs). Remember to use high-contrast text, even when the text is actually in an image.
Colors should pass WCAG 2.0 AA standards for color contrast.