Red skin on tail, preen gland, and butt...no feather loss

[Hend6475]

My Australorp rooster has really red skin all around his tail. There is no feather loss but his feathers look dirty and maybe a bit oily. I first noticed after a cold snap down to -12. While he is in a coop with 7 other birds, he is in a room by himself because I have another rooster. I thought the cold could've given him a rash but it doesn't quite make sense especially since his comb looks fine. I tried but was unable to get a photo

 

[Aapomp831]

Is the redness around his vent?

 

[Hend6475]

Yes, but also on top of his tail. Also, when I say his feathers are dirty I dont mean poo, I mean they're oily and collecting dirt because of it