Single Action: Refers to airbrushes on which the trigger controls only the airflows. When the trigger is depressed, a preset amount of fluid is sprayed. The amount of fluid is regulated by turning the needle adjustment screw at the back of the handle, or, in the case of an external-mix airbrush, by turning the fluid cap on the paint tip at the front of the airbrush.
Dual Action: Refers to airbrushes on which the trigger controls both air and color (down for air, back for color). This simple maneuver allows the artist to change the width of the line, the range of value, and the opacity of paint without stopping their hand motion.