Going to walk through the process I did to make T. Hawk.
Get a solid drawing.
Polygon lasso and flood fill drawing and keep separate colors as separate layers.
In a Softlight layer I painting in facial banding. Warmer nose and yellower forehead.
In a Screen layer light the scene with key directional light. I made a sunlight. i did this by airbrushing in an orange/brown into the screen layer.
Turn off sunlight layer and make another screen layer for ambient lighting. In this case, since the sky is blue I lit him with blue lighting.
In a Multiply layer I paint in ambient occlusion. With all of these layers I can HSV slide them until the color combo feels right.
While I going through steps 3-6 I have all other lighting turned off. This is so I can light the subject in a distilled environment without getting confused. When I am done I can turn them all on together and see if I am getting good results.
Now I take my photoshop file into Painter and work on edges and subsurface scattering.