I might check to make sure it isn't blood, maybe from a fight, though it doesn't look like that from the pic. If not, I agree with KYT, probably stress, excitement or even illness. When some of my birds get ill their comb usually turns a pale or dark purple, depending on how bad it is. If you can catch him without getting spurred I would feel his comb. If it is really cold or really hot that is usually a sign of sickness or shock of some kind. See if he seems to have any pain when you touch it.
What is the temp by you? If it is cold by you and this is his first time out in the cold it could be shock from the cold or mild frostbite. In that case, keep an eye on him to make sure it goes away, but it will probably be fine once he is acclimated. Just make sure he has a place to go that is sheltered from the elements.
Hope he get's better!
