Hey there! I'm afraid I can't be of much help on this subject since it's mostly based on intuition and experience.
My process of choosing colors is mostly iterative, I'll usually "guess" a few colors, and slowly tweak them as the piece develops, until I'm satisfied.
I believe focusing on certain feelings/imagery you'd like to express really helps. For example, on this piece geminimax.deviantart.com/art/E…
, I focused on the feelings of loneliness and desolation, so the palette consists mostly of colors with very low saturations and dark blues(I think it's pretty obvious that I love the blue color).
I can go a little more in depth about my colors choices in certain pieces if you want, but like I said, it's a very subjective thing. Don't be afraid to experiment with different/bizarre palettes!