This may be from either poor circulation, dehydration or parasites you mention the hens are also effected this lets me to believe its parasites and Iam not sure what type. Could be lice mites

chickenbreedslist.com

I've noticed that the tips of my cockerel's comb have become slightly discoloured and wondered what would cause this? He is fine in himself and it can't be frostbite.

I also noticed some of my hens get quite dry flaky combs and wondered what caused that and the treatment?

Thanks :)