Here’s a quick look at what my process looks like nowadays! I really only use the default pen settings in sai and the air brush for most of my coloring. Once everything’s merged, I use this brush:
to fix up some shapes which I definitely see better once all the colors are laid out. As for color palettes/theory, that’s still something I’m figuring out for myself, but don’t be afraid to rely on photos/pre-existing color palettes to test the waters!