Separate her so you can check her poops and administer one-on-one treatment, might be the best bet.
Yellow skin likely equals jaundice (or liver failure) which can be from any one of many, many potential causes. Not eating is often a good sign the liver is suffering because it makes them nauseous and they can't digest food well while their liver's under the weather; this is a good thing in the short term because a failing liver fails faster when forced to digest food instead of repairing. It can do one or the other, not both, so a fast is necessary for total health in normal animals as well as sick ones.
I haven't seen her poops but if the white is yellow and the black is nonexistent or green, and the whole thing's runny, then it's blackhead. But liver failure can be caused by various things. The vast majority of chickens die from gastrointestinal disease, especially liver disease. It may not be that or it may not have started as that but a yellow face shows the bile is not being excreted properly but instead is tainting the bloodstream; her liver's not cleaning the blood properly anymore. It makes the animal ill, confused, increasingly toxified, and requires treatment because once it's reached this point it doesn't get better by itself.
As to treatment, I only use natural methods (though I would use surgery if needed to) because people in my family are sensitive to antibiotics made by man, so we have to use natural alternatives. I can't advise you if you want to use pharmaceutical drugs for this reason. I don't have any experience with them. For a failing liver I would put the animal on a diet of raw grated potato with honey, and maybe stuff like dandelion or milkthistle. The raw potato can support a failing liver indefinitely, even with severe viral hepatitis, and the dandelion or milkthistle, which are more or less identical in their active chemical constituents, allow the liver to heal itself.
Best wishes with your bird.
