Always. In the session I played yesterday the rogue was being racist, so I painted a bulls-eye on his back while he was asleep. We walked all the way through a town and were drinking at the tavern before he final figured out why all the townsfolk were snickering at him.
Even better is that when he found out and tried to remove it, the DM said it had been too long and the ink soaked into his armor. His 'fix' was drawing an elf's head in the center of the bullseye. Antics will ensue...