ionone,
i dunno if this helps but my mate got a hold of some fabric dye a while back. It smudges easily, you only need spit to smudge it but it leaves a stain thats hard to get rid of and leaves an excellent ghost.
my mate did a tag on some bus stop, they buffed it but left a stain, bare other people hit it up after that, once all that had been buffed you could still see the stain...