Along with Uba Yellow, one of the most deadly poisonous oil paints known to Man. One general observation is that the very existence of such paints - as Moist von Lipwig reflected, arsenic comes in a rainbow of colours all of its very own - would make art lessons more interesting, problematical, and professionally relevant at the Assassins' Guild school.
During the Disc's one and only successful space flight (as opposed to missions directed from Krull, the astronauts actually returned), this caused a certain straining of relations among the astronauts. In a moment of rare lack of foresight by mission director Leonard of Quirm - well, not even a genius can be expected to think of everything - zero-gravity rations, designed to be eaten straight from the container in deep space without floating off the plate and making a mess everywhere, were packed into exactly the same sort of tubes into which Leonard had packed his oil paints.
This resulted in Leonard having to paint his on-site moonscapes with, for instance, creamed broccoli standing in for green paint, while Captain Carrot Ironfoundersson ingested just enough Agatean White to necessitate his testing the on-board privy to its limits.
Up until fairly recently on Roundworld, it didn't help matters much that often-intrinsically poisonous artists' oil paints were packed in tubes made out of a material that had the ideal flexibility and malleability - lead foil...