In the past, I had a number of bantam hens in an unheated pen in temperatures below freezing (including a few days below freezing, not just nights.) I did not notice any real problems, except that I think maybe one hen lost the tips of her comb to frostbite. Breeds included OEGB in several colors, bantam EE, Dark Cornish bantam, I think a Barred Rock bantam or maybe some color of Wyandotte bantam.
But I have not yet had them in a place with really cold temperatures for a long time. So I'm not sure at what point they might start to have trouble.
Hmm, I don't know what happened there. What color were her breast feathers before they were salmon colored? Were they a different color, or just not there at all?