Freehand camouflage with a very fine line at the demarcation between colors is the single hardest thing to do with an airbrush. It isn't easy, the ability doesn't come overnight, but with practice it can be done.
1. Thin paint, low pressure, very close to the surface (as in less than 1/2")
2. Angle the airbrush about 45 degrees so that overspray goes into the color that you are painting.
3. If you get too much overspray load the other color and go back and touch things up in the same manner.