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: