Is she laying? If she isn't her cone will naturally go paler.
Otherwise there are various things that can make her anemic like lice and other parasites - not much help but I would give her liver or another high protein food (cooked to kill the germs). Dried mealworms or other insects are good too.
I have one phoenix hen like this about a year old but ever since I let her hatch her chicks she has been staying mostly with them, she doesn't go near the main flock at all, she had lice when I first got her and when I eventually got rid of them she was extremly timid and lathargic, the males just squished her so I kept her in isolation, she went broody so I let her sit and hatch her eggs and though she is still antisocial she's alot better now, even with her pale comb.
You could check if shes egg bound - I've heard about it but I never really had a problem so I don't know much about it but the symptoms might match...maybe...I don't really know.
Hope this helped to some limited extent...
