Frostbite typically starts with a whitish gray scalded appearance and turns black as the tissue begins to die. If they are burnt enough, that black section will eventually fall off. Straight combs are more prone to this issue because the rooster can't get the tips of the comb under his wing during the night.
http://www.freewebs.com/professorchickensdiseases/other.htm
Jody
http://www.freewebs.com/professorchickensdiseases/other.htm
Jody