One of the main differences between types of airbrushes is the location of the color cup is located. A gravity feed style has the color cup above the airbrush, which encourages consistent flow. A suction feed style has the color cup below the airbrush, which allows the color to be changed easily and frequently. A side feed style has the cup to the side, allowing a large range of motion. There are also single-action and double-action airbrushes.
An airbrush requires a tube for the air and paint to mix. This tube must be hollow and long enough for the user to hold securely, like a pen. An empty mechanical pencil or pen is an ideal size and shape for this. In addition, the plastic is easy to drill holes through so that you can add tubes for the air supply and color cup or even add a way to regulate the pressure and control the air flow to the paint.
The color cup is a device that holds the paint while using the airbrush. You can use almost any small, waterproof container for a color cup, such as an empty ink bottle, photo film canister or medicine bottle. Because it does not need to attach directly to the airbrush, a suction-feed airbrush is the easiest design for making your own airbrush. If you use a pen for your airbrush, you can empty out the ink and use the hollow tube to connect the color cup to the pen.
An airbrush, as might be interpreted from the name, requires air to function. However, depending upon the design of your brush, any source of compressed air can potentially propel the paint. You can make your own air compressor or buy one. For a less expensive and more lightweight option, you could purchase a can of compressed air.