Hi, welcome to BYC!
Sorry your pullet isn't feeling well.
I've never really noticed a yellow face in chickens. Not sure if you can see it on the ones with yellow skin... is that a possibility, that it's just her skin color never noticed before?
I'm sure you are right if you think something is off.
Any more info like is she laying and do you know when was the last time? How has your weather been? What are you feeding including treats and supplements? Any recent changes? How many birds have ya got total in how much space? Are you able to get a pic of her droppings or describe them? Like maybe look under where she roosting? Maybe a pic of her too? Have you been able to pick her up and check her over to look for parasites. When was the last time she ate or drank anything?
First thing that came to mind was when babies get jaundice. It may not be relative, but very well could be as I've seen a lot things that can be the same. Here's one link I pulled, just brain storming...
https://www.medicalnewstoday.com/articles/165749.php
Chickens require fast action. Hopefully you will get a lot of response. But answering these questions will help folks to access more quickly what might be the issue. There is no wrong answer you can give. People just wanna help!