One of the biggest advantages of using an airbrush is that you can use it to apply just about any type of paint there is. Watercolour paints, for example, are one of the most popular paints for use with airbrushes because they are easy to thin down and don't usually get clogged up in the airbrush's intricate parts. Both watercolour paints and airbrushes are ideal for making smooth gradients and blending colours seamlessly. Airbrushes also apply the paint evenly without leaving brush marks or drying lines.
Consider the type of watercolour paint you will use with your airbrush. Watercolours come in cakes and tubes. Cakes are dried, densely packed powders which can be moistened to create liquid paint. Watercolours that come in tubes are concentrated to a paste that should be thinned down with water. Since you need to thin the paint down sufficiently to use with an airbrush, tube watercolours work best.
Choose the right paper for your painting. Because of the moisture contained in watercolour paint, it is important to choose paper that is properly treated to hold this medium. If only watercolours will be used, watercolour paper or Bristol board can be used. If you plan to use other media -- pencil, ink, etc. -- you'll want to go with a sturdier paper like illustration board.
Thin the paint down and fill the airbrush. Watercolours should be thinned down to the consistency of apple juice. The paint should be translucent enough so that you can see through it. Also make sure that there are no clumps of paint that can get caught up and clog the airbrush.
Spray the watercolour through the airbrush. After turning on your air supply, pull the airbrush's trigger to release the paint. The tip of your airbrush should be about three or four inches from the surface you are painting on. You should spray in a sweeping motion, making sure never to spray too much paint in any one area as this will cause the paint to pool up and drip.
Use layers to increase the intensity of the colours. Watercolours should be applied in layers of thin, almost transparent paint. Whenever you want to add intensity and opacity to your painting, increase the number of layers of paint you apply to specific areas. If you really want to increase the intensity of the colours, wait for layers to dry completely before adding new layers.
Use high quality watercolours. Cheaper paints are inconsistent in their colours and are hard to mix properly.
Make sure to properly clean your airbrush between uses. Small chunks of dried up paint can get stuck inside the airbrush and cause it to clog.