Multicolour shading tutorial
This tutorial is for Photoshop 5.0, but works in other versions of Photoshop as well.
 
1. Draw the outline and the filled shirt on two seperate layers. 2. Create another layer and with a filled area. 3. Start by drawing the dark shading.
     
4. Then draw the light shading. 5. Smudge. 6. Remove everything outside the shirt.
     
7. Sharpen, if you want to. 8. Done!