If you want to play with light and shadows on model aircraft, you have two basic options: pre-shading or post-shading.
Post-shading is more demanding, because if you are not careful with the airbrush you can ruin the paint job. Pastels can be used too. But they are tricky, because the effect may be different when sealing the model with clear coat.
A simpler alternative is to post-shade by spraying panel lines with quick, thin lines of very thin black paint -- say 90 percent thinner and 10 percent flat black. After three or four passes, you will notice that the panel line is softly darkened, just like with pre-shading. It is very useful on complex camouflage patterns, because any pre-shading can be lost under multiple coats of paint.
Tip submitted by:Fernando Moreno VillaSanta Catarina, Nuevo Leon, Mexico