Q: How do you paint a rabbit purple?
A: With purple hare spray!

Explanation: Hair≠ Hare  but they sound the same.  Hair is on your head and sometimes people use hairspray to keep their hair in place.  Hare is an animal similar to a rabbit.  And there is no such thing as Hare Spray!  (But there is spray paint.)

That is what makes this joke funny- playing with the word hare/ hair and the idea of purple hare spray to paint a hare the color purple.

All of this reminded me of the movie Hairspray so here is the final song: