What do you use to mark your scope ring nuts after they have been torqued to the proper inch pounds to see if they have loosened? I think a grease pencil / china marker would work...
I use Elmer's paint markers from the craft store. You can get them in single colors of your choosing or in a multi-pack. I have a bunch of colors and use them for all kinds of other stuff like painting front sights and highlighting roll marks. They dry pretty quick and have been durable enough that I don't have to touch them up after every comp or range trip.