There has to be enough contrast between the background color and the color of the text on a web page for people to read the text. Some people with disabilities need more contrast than most people.

The World Wide Web Consortium (W3C) has guidelines for contrast in the Web Content Accessibility Guidelines 2.0.

The specific part of the document is 1.4 Distinguishable: Make it easier for users to see and hear content including separating foreground from background.

Gez Lemon, who is in the WCAG working group, has a colour contrast analyser tool you can use to measure the contrast between your text and background colors. Gez did have a Firefox extension. Check to see if there is a version for the current browser version.

Steve Faulkner's Web Accessibility Toolbar for IE may still have the colour contrast analyser tool included in its features.


Resources:

10 colour contrast checking tools to improve the accessibility of your design
http://www.456bereastreet.com/archive/200709/10_colour_contrast_checking_tools_to_improve_the_accessibility_of_your_design/