At first, I painted artwork in black and white. When finished, I created new layer then I used clipping mask to paint color. I have known this trick for a long time from youtube but i don't think it quite good. For now when i used this trick, i realize it's saving me much time of working with color palette and i can focus on blocking with shadow and highlight. However I think my artwork is too much contract, this may because I used clipping mask trick unskillfully.