It goes something like this

likeness, expression, and harmony.

In likeness you paint it to make it look like something else.

Expression is using the colours and quaLITY OF LINES to make it express a feeling, however subtle or cliched (red for anger, or red in a ironic way amongst a pattern of happiness)

Harmony is the aesthetic principle. The attempt at balancing colours, picking colours for the purpose of harmony and of what looks right and beautiful.

I have no intention of painting abstract, so likeness is important to me. All three are motivations, and it's difficult to pick one over the others. Do i paint the jumper green because it expresses something only green can do? or do i do it because there's a really nice red there in the background which would set it off nicely.. It depends, usually it won't matter what the colour of the jumper really is; i'll pick a better colour. So likeness is probably least of my concerns when dealing with colour. But Likeness is definitely important, without it the harmony and expression would have no meaning. What would red mean if it didn't have a jumper or a tree or a landscape to cling to? It would mean nothing specific at all. And if it's an attempt at grandiose generalisations about the nature of man or art, or whatever, it usually blunders into cliched territory, so no, i'll stick to specifics.

In picking colours, harmony and expression don't necessarily fight it out amongst themselves, as compromise. That face needs to be shaded with red, or black, or blue, because that expresses (not what i feel while painting it, but) what the character or landscape or still life is meant to express. So maybe it needs a red outline, but because that red is there, it means i'd like a light orange over here next to this green, not because that orange expresses something, but because it helps make the complimentary "one-two" of red and green less obvious.
It goes something like that.