This question has been asked and answered several times. I say 'go to a hobby shop and look in the model paint section for Testor.' Comes in many colors, flat, glossy, metallic and can be applied with a small brush you can get there. While this may not handle a paint job that graduate (morph?) from one color to another, you may be able to get the start/end colors right.
Testor paint is about $1.50 for 3 Ml, but that's a lot of paint.