Just been researching the different types of colour-deficient vision... very complicated. It's something that web developers ought to pay attention to, but it tends to be a bit of an afterthought, until you realise that 8% of men have some form of colour deficit in their vision.
This site has the best explanation of the different types of colour deficiency (commonly but incorrectly known as colour-blindness): http://webexhibits.org/causesofcolor/2.html, and it has lots of interesting stuff about colour as well. There's a shorter explanation at www.vischeck.com/info/wade.php.
On this site you can check your images to see how they look to a person with colour deficient vision: www.vischeck.com. You can also "Daltonize" them (convert them to a colour-deficiency accessible version).
There are three different components to colour: Hue, Saturation, and Lightness. If you need to use colour to convey information, try not to use colours of the same saturation, and include other information like shape or hatching to distinguish between different objects. The BT website has colour charts showing the effects of the different types of colour-deficient vision.
Another option for web page designers is to offer different CSS stylesheets for different visual impairments. I have implemented this on my work homepage, and you can download all the necessary bits in the form of a ZIP file.
Oh, and I found a site in German: www.ichbinfarbenblind.de/
- I especially liked the section.