WCAG 2.0 - Colour Contrast

http://www.w3.org/TR/UNDERSTANDING-WCAG20/conformance.html#uc-levels-head

Conformance Level: One of the following levels of conformance is met in full.

• Level A: For Level A conformance (the minimum level of conformance), the Web page satisfies all the Level A Success Criteria, or a conforming alternate version is provided.

• Level AA: For Level AA conformance, the Web page satisfies all the Level A and Level AA Success Criteria, or a Level AA conforming alternate version is provided.

• Level AAA: For Level AAA conformance, the Web page satisfies all the Level A, Level AA and Level AAA Success Criteria, or a Level AAA conforming alternate version is provided.

Note 2: It is not recommended that Level AAA conformance be required as a general policy for entire sites because it is not possible to satisfy all Level AAA Success Criteria for some content.

http://www.w3.org/TR/WCAG/

A: (1.4.1 Use of Color) Color is not used as the only visual means of conveying information, indicating an action, prompting a response, or distinguishing a visual element.

AA: (1.4.3 Contrast (Minimum)) The visual presentation of text and images of text has a contrast ratio of at least 4.5:1, except for the following: (Level AA)

• *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.

AAA: (1.4.6 Contrast (Enhanced)) The visual presentation of text and images of text has a contrast ratio of at least 7:1, except for the following: (Level AAA)

• *Large Text: Large-scale text and images of large-scale text have a contrast ratio of at least 4.5:1;
• *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.

Resources:
http://www.visionaustralia.org/info.aspx?page=628
http://snook.ca/technical/colour_contrast/colour.html
http://juicystudio.com/services/csstest.php
http://wearecolorblind.com/