Guideline 2. Don't rely on color alone.
Section 508, Subpart C (Technical Standards) 1194.22
http://www.w3.org/TR/WCAG10/#gl-color
"If color alone is used to convey information, people who cannot differentiate between certain colors and users with devices that have non-color or non-visual displays will not receive the information. When foreground and background colors are too close to the same hue, they may not provide sufficient contrast when viewed using monochrome displays or by people with different types of color deficits."
Checkpoint 2.1 - Ensure that all information conveyed with color is also available without color, for example from context or markup.
Checkpoint 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.
In a nutshell when testing your web site, you need to make sure you test the look and feel of your website from a non-color perspective. Avoid using Make sure that your font colors, background colors and images are controlled by your CSS document. This way, they can be easily disabled. One way to test your site for safe color schemes is to view the site with a monochrome monitor, or disable the colors in your web browser. Navigate through your web pages to make sure that the contrast is high enough between the fonts and background colors, so that those with color deficiencies will be able to see the content on your site.
Another way to test your website is to print out a few pages uses black and white printer (with colors and images printing in grayscale). After printing out a page, photocopy it, then photocopy that copy and repeat 2 or three more times. If your page is still readable, you’re probably doing okay, but any areas that fade too much will need some secondary methods. For example, don’t just have all the links in your website appear a certain color. Make sure that they are underlined as well for added clarity.
When designing your website avoid using foreground and background colors that are too much alike (e.g. blue background with green text). Be sensitive in your design to allow all visitors the same viewing experience by making your page content clear and visible.