If it's to give you a clue which string is which in plucking, You might try a permanent marker like a sharpie that is transparent on metal and isn't heavy like paint to dampen the strings. I'm not SURE that it won't clog/dampen, but trying it out on a seldom used string is how I'd do it (like a G#). For visual clues, you don't necessarily have to color up the entire string, perhaps bars ans spaces. I suspect the marker will wear down easily, too. Might try this out with my third in like Autoharp to see how it works. The idea intrigues me.

Joanne in Cleveland