I use the little bottles of touch up paint that you get at auto stores. Clean the metal with a solvent like acetone, then carefully brush in the paint. The trick is in the removal of the excess around the edges. If you wait to long, it's bonded to the metal and you have to use a solvent to get it off. If you do it to soon, it smears the whole thing. I use a magnifyer and a very sharp metal probe. Carefully, pull the excess away from the part you want to keep. If you try to peel it towards the keeper stuff, it'll all peel up. Another trick is to use a toothpick for the application instead of the brush. Push it into the stamping and let the paint wick off of the toothpick into the channels.
Final touchup can be done with a flat edge with a cloth wrapped around it and acetone. Use the flat edge to keep the cloth flat on the metal and wipe off the excess paint. Note: If your finish is duracoat, it will smear from the acetone.
Here's one that was done with a fake gold inlay. This stuff is more like a wax and is really messy. Once clearcoated, it's fine.
Be sure to coat it with clearcoat. Keep yer powder dry, Mac.
Tuff-Gun Finishes. The Name Says It All.
Mac's Shootin' irons.