colored lettering inset into etched guns are usually rubbed-in by use of a lacquer pencil. It's like a crayon.
The area is rubbed with the pencil, color fills the etched lettering, proofmark, logo area, and any excess on the outside can be wiped-off with an alcohol pad. The lacquer is supposed to harden and stay, but modern solvents will take it right off. Your lead-away cloth will do so.
For touch-ups, just buy some from Brownells for ~ $2.00,
LAQCUER-STIK stock no#517-100-002