Airbrush Painting Camo

It depends on how soft and edge you want.  You can use lengths of rolled up tape just inside your mask edges to lift the edge of the mask a bit.  With som experimation on air-pressure and distance form painted surface, you might be able to get the right effect.  I have used that method prior to having a controllable airbrush, and it worked somewhat.  The effect wasn't so much that of a soft transition as that of simply taking a bit of the starkness from it.

What kind of airbrush do you have?  Even a very simple single action like a Paasche H can easily do fine enough lines to allow you to do free-hand camo, but like everything with airbrushing, it is a matter of practice and experimentation.