Apply a solid base coat that is close to the flesh tone that you are trying to recreate. Some colors that you can choose from include Sand, Dark Brown and Peach. You can also mix colors to achieve a closer match to your desired flesh tone.
Softly mottle reds and purples onto the base coat to begin adding variety to the flesh tone. To do this, thin down the paint a bit with distilled water and spray translucent little whirls of color. Keep them light and random. You want the base coat to show through. Remember that you can always add more, but taking paint away if you overdo it is much harder.
Spatter the same red and purples onto the canvas to create freckles and additional variation. This is accomplished by turning down the PSI, removing the needle cap from the airbrush gun and flicking the trigger to produce spatters of paint. It takes some practice, so you might want to try it on a piece of paper before committing it to your painting.
Add yellows, browns, blues or greens as needed following the two techniques already described. The exact colors you use will vary based on the skin tone that you are trying to recreate.
Add veins by spraying a very thin blue-green mixture. The veins should be barely visible so as not to draw attention to themselves.
Lightly add another coat of the base tone to join the colors together. You can mottle or spatter them depending on the amount of coverage needed (mottling covers more than spattering).