Here is an interesting page showing some freehand camouflage. Be sure to click on the pictures of the Spitfire to blow them up. If you read the article you'll find that it is 1/72 scale and the camo was done freehand with an external mix,single action Paasche H airbrush. Admittedly, the modeler, Roy Sutherland is a pro and has spent many hours and gallons of paint learning the art. But, what strikes me is the fact that,even though he has probably tried many different airbrushes, he chose the simple Paasche H for this task. Of course, he could be just showing off. 
But it's a good build article anyway.
Don