im gonna be honest my color picking process is just trial and error haha so i cant tell u much if im doing a background piece ill usually choose the colors for the background first and then paint the characters to match. i think the most important thing with colors is not getting fixated on the details and  just loosely putting the colors on ur canvas first to get a general sense of what mood/lighting u want to get. i also usually turn off my lineart layer to focus on the shapes instead of tiny details ^__^ other than that . i just do whatever looks nice honestly a good chunk of what i do while drawing is experimenting + hue shifting and i tend to switch up my colors a lot anyways