In this Tutorial I will show you how to shade. I'm going to make a Simple Circle as an example.
Step 1: Draw your outline. I recomend using black.
Step 2: Draw platelets of your primary color getting drawer and lighter on the side and color the object with your primary color. After, press the paint bucket and press shift+click on the black with your darkest color. The yellow in the picture is going to be our light source.
Step 3: Take your second darkest color and put it on the side farthest away from the light source, then take the lightest color and put it closest to the light source. If you want, you could be finished right now.
Step 4: How to dither: Take your second lightest color and put it near your lightest color in a checkerboard type pattern. Do the same for the Third darkest color.
Step 5: How to shade: Take your second lighest color and put it next to your lightest color do the same for the third darkest.
Finished Product: