This is something I find quite hard without seeing a car in the colour and seeing how the light plays on it etc.
Found an free app on the iPad which helps a bit called ColourEffects. The pictures below have lost a lot of the quality when moving them around but they look a lot better viewed within the app on the iPad.
Changing the colour takes seconds but selecting the exact colour you want is harder. I know it isn't photoshop but it is quick, free and easy. Spend a bit of time and you will get much better results that the ones below.
And just to show you a REALLY bad colour choice I give you gold chrome