In the days before computer coloring, cartoonist used airbrush to color mock-ups of their work. While airbrushing is too tedious to use in every frame of a true animation, it works well for coloring and shading cover art and cartoon posters. The main benefit to using an airbrush rather than traditional paints is the quality of shading that the artist can achieve. An airbrush can produce even, uniform gradients of color that brushes cannot reproduce.
Select a background color from your paints and fill the reservoir of your airbrush with it. Connect the hose of your airbrush to a canister of compressed air. Point the airbrush at the poster board and squeeze the trigger. A fine spray of paint should color the board. Cover the board in an even layer of background color. When finished, rinse the airbrush canister and mechanism in warm water to clean it of paint.
Sketch your cartoon lightly on top of the background using a pencil. Take your time and use reference pictures if you need to for difficult poses. Erase lines that become too cluttered.
Lay down several layers of newspaper over your work surface and don a face mask to keep from inhaling paint. Place a piece of posterboard on top of the newspaper.
Shade your cartoon. Decide on where your lighting is coming from. If it is an outdoor scene, this will be from overhead. Select slightly darker shades of your base colors and spray the side of each character and object in your cartoon that is furthest from the light source with your airbrush. Use light, short squirts at first to build up layers of darkness until you achieve the effect you wish. Clean the airbrush whenever you change colors.
Edge around the outlines of your sketch with masking tape. This will prevent your airbrush from coloring outside the lines when you begin to fill in your sketch with paint.
Paint the flat colors of your cartoon. The flat colors are the basic colors of your picture, neither too dark nor too light. To do this, select your first paint color and fill the airbrush reservoir with it. Aim the airbrush at the sketch you wish to color and squeeze the trigger. When you need to change colors, clean the airbrush in the sink and fill it with new paint.
Select lighter versions of your flat colors. These will serve as the highlights in your picture. Go along the edges of each object and character in your cartoon that is nearest to your light source. Clean your airbrush every time you change colors.