Yes, I too use a Sharpie and in addition, I also use any left over Cold Blue, I happen to have around. ....
I routinely rework of assemble Field Rods for traditional M/L's and to give it an old age look, I carefully apply some Cold Bluing. It turns the new brass a dull brown and suspect that is basically what the specialty products have in them. I believe it's nitric acid. Not sure how this might play in your usage but you can always spot it or whatever. Just be sure to wash off the excess. ...
Be Safe !!!