Well, there are plenty of ways to go about that (as far as I know) And if I understand you right, then the easiest way to go about it is to:
1. Select the faces you want to be darker using the point/edit mode.
2. And by right-clicking all of the faces you want to be darker, (you may want to spin the model around every now and then by hitting "ctrl+r" so you can see the rest of the faces you want.
3. Then going to the materials editor once you have the faces selected, and make a darker material.
4. Then apply that material to those faces by clicking "apply" when all the faces that are supposed to be darker are selected.
helpful hint: you must have the "face" select once you're in the point/edit mode. if you are in any other select mode, then it won't work right.
sound clear enough?
I learned that from the manual btw. it's in the materials section, I think.
hope I helped
