Both pan watercolors and those in tubes can be thinned with water for airbrushing. When using pan watercolors you can lather the paint with a paint brush and then transfer it via the brush to the airbrush color cup (or reservoir) for spraying. Tube watercolor, the type most commonly used, can be thinned with water in a cup or jar and then poured into the color cup.
In many cases an acrylic painting is done with a combination of techniques that incorporates anything from screen printing to sponging. All techniques, including airbrushing, work well together when done in artist acrylic colors.