The top edge?Just along the edge of his comb right now.
Does he have a large single comb?
Grayish or yellowish is mild frostbite and will likely not be lost.
Necrotic(dead) tissue will turn black and fall off eventually.
Here's some pics of frostbite progression:
https://www.backyardchickens.com/articles/frostbite-in-sw-michigan.74597/