There's also a techniqe known as "pre-shading".
What you do is mix up a darker version of the color of the final coat. Let's say your final color is grey. Mix up a batch of very dark grey and spray it wherever you would like to see tonal variations. Some people do it to the center of each panel, some, to the panel lines.
Either way, spray where you want the variation to occur. It can be symmetrical or random. After it's dried, spray down your top color, but spray in light coats. Stop when the under-color is just barely visible through the top color.
-Fred