Use the shaders for lighting and shadows. The shaders that can be used for lighting is the white shader and the gray shader. Warm shader can be used for shadowing.

Here is a before and after using shaders.

Without shaders.

With shaders.

You can do this by lassoing what you want to shade.

Here's another example. I used the white shader that can see what colors should be black and what colors should be white. Then I used the grey shader next to the white shader. I imagined the light coming from the right. So I went from right to left chromatic and then light/grayish. I'll first show you the original picture and then the shaded picture and you can decide wether you like this way of shading or not.

The original picture without shaders.

The picture with shaders.

We can of course also exclude the white shader which is chromatic to only use shadow and light.

Finally, you can use the blur brush on the painting and there you have it, a bald orc avatar.