This is nothing more than a transparent wash of dark on the shaded side and a wash of white on the lighted side. But because of the base texture you get the illusion of depth. 90% of the work was done with the random splattering of paint that we did in this post
Same thing here. Let the colors and textures of your base painting show through most of the work. People will be very impressed with the exquisite detail you put into these skulls when it's actually just a thin wash of paint.