From what I've read it could be he's stressed, excited or possibly a circulatory problem. Rooster's combs can turn purple when they are excited or stressed. If he is excited, it should go back to normal and will happen intermittently. If he is stressed, it might signal a circulation issue.
Also, you may want to check to see if he's getting picked on either by the hens or another roo?
Either way, I would keep an eye on him and see if it progresses.
I hope this helps a little