Mort,
I assume you are doing a NATO scheme? Now, what I'm going to tell you depends quite a bit on the quality of your airbrush and set up. In other words, I'll just assume you have a compressor and a regulator. The tri-color NATO scheme will require a defined edge between colors, but not quite a hard edge. Spray your base color over the entire model first (NATO Green) and let it cure. Then begin with the secondary color (NATO Brown?) and spray the outline of the pattern first, then fill in. Thin your paint to about the consistency of 2% milk, and draw the outline of the secondary pattern. If you are using a single action brush, I would go down to about 10psi, get relatively close to the work (3 to 4 inches) and remember to keep the brush moving. Later, when you are ready to fill in the pattern, you can dial up the preesure some, and you will be well away from the borderline and avoid the overspray,
If you have a double action brush, go with high pressure (20psi) and use the same outlining technique, controlling paint and airflow with your trigger.
Use the same technique when you apply the NATO Black.
My suggestion is that you use acrylic flat (Tamiya or Gunze) paint. The techniques above will work well with enamels. With parctice you can get results like this:
Good Luck, -e-mail me if you need more help;
Steve