I've used small bottles of gold model car paint. (Testor's)
I first degrease the area with laq thinner, then flow in some paint with either the tip of a toothpick or Q-tip (don't worry about overfill).
I then very slightly dampen a facial tissue with the laq thinner, and very lightly wipe off the excess gold paint from the high/flat areas, being careful not to press hard enough to drive the tissue down deep enough to scour out the paint in the stamping/roll engraving.
After letting the paint dry for a few hours, I run a lightly oiled cloth over the degreased areas around the treated area(s)
.