It's when you add to the visual effect of an existing work of art through airbrush technique. It is done on drawings, prints, and photographs as well as other works on paper. An example of this technique would be the black line drawing of a caricature artist who is working at a theme park.
Color is added to the face and hair using the airbrush in a freehand manner.
Usually the artist purposely makes no attempt to keep the color inside the lines. Any style of airbrush can be used, but the internal mix airbrush is preferred because it produces the softest spray. And all air sources can be used from a propellant can to a compressor or compressed gas. Although you want the ability to regulate the air pressure and to work between 5psi to 20psi, you do not want to utilize pressure that is too high because of increased overspray. In the case of paints, waterbased paints are the easiest to work with and are the lowest in toxicity.