Hi there, welcome to BYC!
If the vet wasn't able to identify an injury via swelling, heat, or pain response.. The answer is likely Marek's. Not all birds will present with the same symptoms. Many folks are in denial about it happening to them. Here is the most valid information found..
Marek's Virus FAQ
There are a number of things can present similar to Marek's and laboratory testing may be the only way to discern the difference. For what it's worth.. there can appear to be a spontaneous recovery.. the bird is still carrying and shedding Marek's.. it just overcame the symptom of tumor pressing on the sciatic nerve.. Lots of birds survive Marek's. The later in life ones you gotta watch more for slow wasting or things like sour crop. The vaccine would ONLY be HIDING the symptoms IF that's indeed what this is. It doesn't stop birds from getting the Marek's or spreading it. Vaccinated birds not welcome here.. to allow the disease to spread undetected.
Has there been any improvement with the Metacam? Marek's does not cause pain. If it were an injury.. I would expect to see improvement with the metacam... smoother movement, etc.
Otherwise if it were an injury.. people don't often realize that pain and swelling IS the body's way of letting us know there is something wrong and slowing us down by limiting our movement and keeping us from us injuring ourselves further. Pain meds should be used only once the concern is identified accurately.
Sorry you face this.. It isn't as scary as is often let on.
What are you feeding including treats and supplements? I will have to look around for it, but I have seen feather color change being a symptom or result of something specific. Are you able to post photos?
Since she is still laying and active.. I would let her be.. If she gets's worse, consider sending her in for necropsy and confirmation or ruling out as they do microscopic testing. If you think it is Marek's and can't accept possible fall out in the future.. then be sure to only get vaccinated chicks and quarantine while it takes effect. Different strains have differing levels of mortality.
My maiden name was Phillips.
Hope your birds thrive!