What color scratches?

Is the question about the paint rusting, or the surface on a real plane that the scratch uncovers?  The exterior of most aircraft (at least those not fabric covered, were aluminum, which does not rust, but does oxidize to a matt aluminum or matt gray.  There may be small pieces of steel on an aircraft (some hinges, brackets, exhaust stacks, etc., but these are usually pretty small.  So scratches on a real aircraft should range from aluminum color to a matt light gray, most quite matt.  I often mix a shade of half flat aluminum and half flat light gray.  This works out about the same shade as an old aluminum casting.

I've used various acrylic and enamel colors.  Nowadays, I simply use Prismacolor soft core colored pencils.  I use silver, grey, dark brown etc.  

Depends on how "old" you want the scratch to be. So, this could range anywhere from bare steel to bright rust, to dark old brown rust. A combination of colors is going to be the best approach, as you should see a varied depth to the scratch, in other words, some primer should be visible, maybe some vestige of a previous paint job, as well as some bare metal which may or may not have rusted. Guess you will have to decide upon a "story" for your scratches, like how they occurred, how long ago, what kind of environment has the vehicle been in since it got scratched. That will tell you exactly how you need to color them. Have fun!