I’ve never tried it but that’s a pretty good idea, especially if you could buy that blacklight-reactive body paint and a few black lights – and get it to come across well on cam. Alternately, I think they make edible body paint (chocolate flavored, etc.) that might be good if you cam with a partner a lot.
Crossdresser has a good idea about blacklights and reactive paint, but I just don’t know if that would work well via webcam. You need a lot of light for clients to see you and I just don’t think blacklights would be enough. Plus, I don’t know if it would put clients in the right mood.