I agree with Rhodium that silvernitrate is one of the best, longest lasting, hardest to get rid of substance that discolors the skin. About 30 years ago it was one of the initiation rites at the place SWIM study to paint a certain symbol on the foreheads of the freshmen. That's not done anymore, for some reason...
During my latest lab-work, I found a great skin-dye: Ninhydrin. It reacts with amino acids to give an intense purple colour. Your greasy palms contain plenty of amino acids to facilitate the reaction, and the stuff is quite potent. It's also become widely employed by lawenforcement to get fingerprints off porous material, where the usual methods, like sophisticated powder-formulations, doesn't work. The nasty stains can be removed with sodium hypochlorite, though.
My hands also underwent accidental localized treatment with the following effective dyes during the last month:
2,4-dinitrophenylhydazine-reagent
Methylene Blue (solution and solid)
Potassium permanganate (can be removed with H
2O
2... And other easier removable dyes/chemicals.
No serious accidents or colorings, just... "chemists-hands", with an assortment of coloured blots if you look closely. But well, they disappear after max a week... I normally only use gloves when handling very hazardous stuff (hate the idea that something might get inside the glove, and I'd have to waste time getting the glove off before flushing), but maybe my GF instructs me to use them when handling skin-dying material as well...
