It really doesn't matter why it's purple. If it slows down the monkey at all, Jamie is happy to know it's there. He heads for a door that leads to the room with the reference desk, trusting the Doctor to be following after.

With luck, there'll be something in there he can use to block the door behind them and slow the monkey down even more, and he's already half-keeping an eye out for anything that would work even as he keeps running.