Staining live worms for individual identification?

Hi,

I’m hoping someone can help me figure out how to stain worms in order to identify live individuals. My basic criteria are that the stain last at least 2-3 days (at which point I can reapply if necessary, although ideally it would be permanent and last the lifetime of the worm), that it won’t rub off on other worms, and that it have no or minimal effects on behaviour, survival or reproductive performance. The staining only needs to be done in males. I read somewhere on here that some authors have used a black pigment mixed with vaseline to make a spot on a worm for use with tracking software. However, I haven’t had much luck getting vaseline to stay on the worms, and I am worried that this could rub off onto other worms. I have also read that it’s possible to feed the worms with a dye, but does anyone know how exactly to go about this and what sorts of dyes might work? I think painting them with a small spot would be preferable, but so far I haven’t found a suitable substance. Any suggestions on that front would be very welcome as well.

Thanks!
Margo