I tried this shading technique in this tutorial on dA.

Before my base colors, I put them all to white and added a clipping layer with the Multiply mode on.

Shaded skin later without the Multiply layer cuz it's too dark and ugly~ xD Manually colored it later with my choice of colors~

Note: The 3rd Link shown on the cover of this comic is all the isolated clipping layers that I colored on. (that's why it's messy~ xD)