I've got an old Colt Diamondback revolver that I got from pops and I use as an ankle carry. Well, it started to show a bit of rust flecks on the barrel despite oiling it up after every carry session. So I did my best to get rid of the rust without ruining the finish, but in the end the finish had to come off to get the rust off. Now I'd like to touch up that spot, but the old Colt blueing is a beautiful black/blue that's hard to match. Any recommendation on what I should use?