For unit markings: I have used transparancy sheets over the original design and then cut out the details with the exacto knife. If there are several colors or designs incorporated you can overlay several different ones to get the effect. Kind of like silkscreening. Touch ups for over spray can be done with a small brush and the base coat color, somewhat like dry brushing.
I have an Aztek airbrush, but like it was said, it is a lot of work. The parent company is Testors, like the old modelors used to use. You need to clean it or your out what you paid for it. Those azteks are around $300 bucks and have to be tended to when your not actually spraying anything, because the get gummed up inside and are useless if you don't. Not to mention you need a small compressor to run them. They do a great job shading though. I have used the canned air pressure type along time ago, but they are not reliable for detailing. Matching colors sucks to. You would have to use one type of paint, either spray can or modelers paint, on the whole thing. it is too hard to match them seperately. Doc