If you're engraving the lines, you can get paint sticks made for
enhancing engraved/stamped marks. Brownells' has lacquer sticks in a
number of colors, I've used them for years. See:
http://www.brownells.com/aspx/NS/sto...aspx?p=7778&s=

I've blued steel, then used the white, I've also left parts in the
white and used the black. White against the blued background seems to
work best for me. The other colors are good for decorative purposes,
not much else. Degrease the metal first, of course. Smear a good
coating of the stuff over the marks, wait awhile, then wipe off the
smeared stuff. Once hardened up, the filled marks are pretty durable.

Keep sealed up after use in some sort of thick pastic bag, the sticks
dry out if you just leave them laying around.

Stan