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!
I can’t find any active ones for Eldar, Dark Eldar, or Necrons though. One suggestion is to use the Dawn of War army designer, and take a screenshot of the outcome.
EDIT – broken painters removed. Sadly no Eldar one is known to replace it.