this is going to be the most difficult part for me as i have low confidence in color use
right now i am experimenting.
at first i tried the flat color you see on the demon, with a plan to add shadows and highlights as needed. but it did not "feel " right
so i grabbed some imageses online that had the color palette i wanted. i them blurred and smudged them down to be only color information and painted the underpainting on the same layer as you can see on the human.
now that i see the flat color with the underpainting i think i will try mixing the two methods