Color Contrast Checker is useful for checking readability of one color of text on another color background, for example. Web usability compliance guidelines, intended to keep the web accessible to disabled people (see http://www.w3.org/TR/2008/REC-WCAG20-20081211/#visual-audio-contrast-contrast) also have guidelines for text contrast, and this tool is an easy way to ensure compliance. Even if you are not moved to do a little extra work to make your site broadly accessible, keep in mind that the number of people with audio and visual impairments is enormous, and you are cutting them out of your potential audience if you do not familiarize yourself with WC3 recommendations.
This calls itself a tutorial but it's more like a fantastic article with lots of useful illustrations. Explains RGB vs. CMYK color very clearly, and then gets into more details of value.
