9. With the "Radial Gradient" tool and the area you wish to color selected. Pull the gradient across the selection starting where the light is coming from and ending near the middle of the selection. This will give you a basic sense of where the light is and establish the direction of the light. (the arrow shows the direction of the gradient on this selection. Starting at the top right and ending near the middle left)

10. Now select the "Lasso" tool. I use the regular lasso for most of the "cuts" areas but this can be done with the Polygon lasso as well. It is important to turn "Anti-aliased" on for the rest of your coloring selections or else you will get some pretty ugly edges on your color areas.

11. With the "Lasso" tool selected make a selection area on or near the side closest to the light source. In this case the upper right. The shape, style and size of this selected area is all part of your style. for the piece I am coloring I am using a slightly textured look so I have made an irregular selection.

12. You the select the "Airbrush" tool with the setting pretty much the same as the radial gradient tool we used earlier. Set to "screen" at about 10 or 15 opacity. Then airbrush along the selection you made, again do this in the direction of the light source. Repeat steps 11 and 12 to keep adding smaller and brighter areas to the colored area.