Research-Based Web Guidelines: Accessibility
http://www.usability.gov/pdfs/guidelines.html

Usability.gov

Abstract:

Ensure that text and graphics are understandable when viewed without color. If designers depend on color to convey information, colorblind users and users with devices that have noncolor or nonvisual displays cannot receive the information. When foreground and background colors are close to the same hue, they may provide insufficient contrast on monochrome displays and for people with certain types of color deficits.