Coloring is just a skill and those train like muscles) One of the easiest methods is to just use an eyedropper tool which makes sense when you already have them established, eventually when copying from already good color schemes you pick up what works subconsciously. And if you do a closed line art it takes a few seconds to fill them in. Pro tip: Do every color on new layer cause that way you can adjust each till you're happy how they look together. Works great on flats, but merge them later. Don't let the chaos of "where's the ******* layer??" happen