i wouldn't do "touch up". i would do an entire re-do with quality epoxy paint of a color that suits your fancy. it goes without mention, absolute protection must be given the optics, knobs, gear tracs, etc.
I would check with GRS or Meji. Go to NAPA,look at the auto touch-up paint.If yo change the whole thing it might be harder to sell,oneday,if needed.God Bless,mike