Colour is used more and more these days to help convey information. When one in twelve men have some measurable degree of colour vision deficiency, the use of certain colours in certain ways can cause difficulty when navigating web pages or software, and even total illegibility in some cases.
The key issue is to know when you are using colours which some people will not be able to differentiate - because that (for them) removes the benefit of using colour for visual cues. Colour scientists have long known which colours are confused by colour blind people, but this tends to be expressed in a way difficult for designers to utilise.
I've done a "translation" of all the colours in the standard 216-colour web-safe palette [refs 3, 11]. The colours may not show you exactly what dichromats see, but will help you to understand which colours are easily confused. This can help you to ensure that important colour cues are not lost.
Engineering Journal, Jan 1999, and the pre-print is available here as a PDF file.
(Go to the Adobe site for a free copy of the Acrobat reader)
This site includes a short Background of colour vision deficiency (10.6k of images), for common terms used here, and a brief explanation. See References for a list of places I've found interesting and/or useful during the information-gathering stage. The Related sites page has links to related tools developed by other people. The information contained on Palette files, Transforming, and Colour choice comprise the toolset.