The only exact match is going to be through complete refinishing. Cold blues have acids that attack the original finish, with the result they change the color around the scratch as well as in it. So you usually have to apply it to the whole gun for even color, and it can compromise the durability of the original finish. An extremely mild cold blue like Blue Wonder might not exhibit this problem, but I haven't tried it.
I would try simply using a magic marker to fill the scratch. At least solvent will take that off if you can't make it look right. Even if it isn't a good permenant solution, you will be able to see whether a slight mismatch of coloring in the scratch itself is close enough for camouflage purposes? If so, you could try the Blue Wonder.
Nick