I like to keep my Interactive Army Painter links up to date, as I find them just so useful as a prelude to getting actual paint on a model!
Bolter & Chainsword has an amazing set of interactive army painters available, letting you plan out a general look and feel for models before you try them. Its no substitute for trying a model or two, but does let you think about your colour choices!
You also have Chaos Marines.
Games Workshop had some OK interactive painters for their old paint ranges, which are gone from their site…. but can be found on the Wayback Machine. The B&C painters are better!
There is a painter program that handles both tau and eldar – its simple to use, but not as flexible as the Bolter and Chainsword options here.
Although its a little over complex and fluffy, there is a great Tyranid painter program here
I still haven’t been able to find any for Dark Eldar, or Necrons though. One suggestion is to use the Dawn of War army designer, and take a screenshot of the outcome.