WCAG 1.4.3 Contrast (Minimum)

(from How to Meet WCAG 2.0 by the W3C) The visual presentation of text and images of text has a contrast ratio of at least 4.5:1, except for the following:

• Large Text: Large-scale text and images of large-scale text have a contrast ratio of at least 3:1;
• Incidental: Text or images of text that are part of an inactive user interface component, that are pure decoration, that are not visible to anyone, or that are part of a picture that contains significant other visual content, have no contrast requirement.
• Logotypes: Text that is part of a logo or brand name has no minimum contrast requirement.

On that last one I wish to point out that if people can't make out what's in your logo, it's a pretty crappy logo. There may be no requirement but don't be an idiot. You want that thing to be recognizable on a poorly-lit billboard in the rain at 70 miles per hour, and that means use some freakin' contrast.

Color Contrast, Beyond the Basics by Beth Raduenzel is a great article about why color contrast is important and also covers why you should underline your goddamned links.

Related Guidelines

1.4.6 Contrast (Enhanced) - Level AAA ratchets the guidelines above up to a ratio of 7:1.

Tools to help meet this guideline

• Color Contrast Check by snook.ca