hen off colour

Have you checked for mites around the vent area?

Best wishes

Christine

Hello, Henkeeper 

As well as the previous suggestions you could check the following :-

Is she eating and drinking OK?
How does her crop feel?
Does she have a temperature?  You can take this by feeling under the wing of one of your other hens and then under her wing to see if she feels warmer.
Is she moving around as usual or is she limping (for example).
Breathing - is she breathing easily as normal or is she "rattling" or sneezing.  is her nose runny?
When was she last wormed?

I am not an expert but this will help us help you.  It may be a good idea to find a local chicken-friendly vet.  Not all of them are and some will even refuse to treat chickens.
In the meantime good luck and keep us posted. 

By the way welcome to the forum, Bossy Boots