The answer to this,.... is that it very much depends on the model. Some models will speak to you, and the second you look at them they shout "hey!!! I need to be green with gold trim",.. or whatever. If the model isn't speaking to you, you may find that it fights you along the way, or just won't come out the way that you see it in your head.
I also try not to paint things exactly like the look on the box. (not that I don't on occassion, because sometimes its a named character from a cartoon or something, which then has to be grey,.. because a pink bugs bunny just plain wouldn't fly,.. for example). Switching up your colors and doing things with your own creative flair is a good thing.
I would say that before you even pick up your brush, sit down and look at the model and choose your 3 basic colors. (lets just assume for a mech/robot).
Primary color: this is going to be the color that you choose for the largest amount of area,.. (In the case of our mech, this would be the color of our armored plates)
Secondary color: this is going to be any "trim" type pieces (ask youself, does this clash with color #1?)
Tertiary color: this one may or may not exist, depending on the model,.. (ask yourself if this clashes with either other color)
Add on colors: this would be all the little splashes of color,.. the brown leather of the gun strap, the silver metal where the hydraulics and other inner working bits show through, the glowing yellow of the eyes,.. etc.
The trick in choosing them is to try to get things that will work well together. I mean,.. having a purple mech is fine, but you probably aren't gonna want to trim it out in neon orange, and then give it a lime green gun. (maybe you would,.. but this combination doesn't work for me personally)
