The Web Content Accessibility Guidelines 2.0 (WCAG) advise that you will need to provide enough contrast between text and its background so that it can be read by people with moderately low vision.
There is no specific contrast ratio required for single A compliance, for AA you need to a 4.51 : 1 ratio, AAA you need 7:1 ratio. For text larger than 18 point or 14 point bold text the ratios are less stringent.
So what is a contrast ratio?
This is great – however you are probably thinking what does a contrast ratio of 7:1 mean? There are a few tools that you can use to check these values, however it is probably worth making sure that they use the WCAG 2.0's luminosity contrast algorithm to ensure compatibility. Most tools will allow you to check the ratios between 2 hexadecimal colours.
http://www.hp.com/hpinfo/abouthp/accessibility/webaccessibility/color_tool.html
http://juicystudio.com/services/luminositycontrastratio.php
The details…
The exact wording for the guideline is:
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.
http://www.w3.org/TR/2008/REC-WCAG20-20081211/#visual-audio-contrast-contrast
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.
http://www.w3.org/TR/2008/REC-WCAG20-20081211/#visual-audio-contrast7
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