What's Purple?

Remember those stupid old jokes, like:

Q: What's green and dances?
A: Fred Asparagus.

or

Q: What's green and sings?
A: Elvis Parsley.

or

Q: What's purple and conquers the world?
A: Alexander the Grape.

Well, I've got a new one for you:

Q: What's purple and a total dickhead?
A: Joe the Plum.