well some diseases cause this but are associated with other more severe symptoms. the other day I was listening to the chicken whisper radio show and they talked about vitamin deficiency. they said PURPLE comb can be caused from a vitamin B¹ deficiency. where B1 is lacking it can actually cause purple comb. so I'd give it a try and get a B1 supplement. I have a great series of B vitamins I give my chickens called vita pro B. its supposed to be readily absorbed by the body rather than expelled out. I got mine from first state vet supply. its worth a try and if it does not work then you know it could be something of a more serious nature. best of luck. hope this helps.