[Twinge]

You want to have enough contrast on your pages' background & foreground colors so that customers can easily and clearly read it. Here's a nifty little tool that gives you a decent idea of how readable your site is, based on a forumla for brightness & color difference:

http://www.snook.ca/technical/colour_contrast/colour.html

Certainly not the only thing you should be using to check out your color scheme (ask people!), but it can be helpful.