1) Buy a Color Wheel from an art store (Michael's) or google one.
http://www.michaels.com/pocket-color-wheel/10270171.html
2) For individual models or vehicles I prime black, and dry-brush the entire model; white, gray, bone, or metal, depending on what the primary color is going to be.
White for bright colors; red, yellow, orange, lavender etc.
Gray for medium colors; blue, green, purple, etc.
Bone for natural colors; tan, brown, flesh (caucasian or otherwise), leather, wood, etc.
Metal for metal colors; silver, gold, bronze, etc.
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First, my old eyes can see the detail better.
Second, the main colors will naturally be slightly lighter on raised areas, and darker in recessed areas.
example;