I regularly mark my male hissers for the same reason. I have used acrylic craft paint in the past, but it seems to wear off quickly. So far, my best solution has been a metallic paint marker. I use a series of dots (on the horns, between the horns and lower on the pronotum) to distinguish between males. It does wear off, but it takes a really long time so I can mark again as needed. So far, I haven't seen any ill effects from using the markers.
I love this idea! Going to go find myself one of those right now.
That's insane... But strangely that sounds very relaxing. I usually use mine to scope out biofluorescent millipedes in the yard ... or adult B. giganteus escapees.