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!
The best painter is for Space Marines, unsurprisingly
There is another one for Terminators.
and for the Sisters of Battle.
You also have Chaos Marines.
The Tau – Fire Warriors.
There is an Eldar one here.
Haven’t been able to find any for Imperial Guard, Dark Eldar, Necron or Tyranids though. One suggestion is to use the Dawn of War army designer, and take a screenshot of the outcome.