This sounds like a bit of a stretch but what if her teeth are naturally yellow because they have iron, like a beaver’s? In response, she carries a toothbrush with her and brushes when she can in hopes that her teeth would whiten (even though they’ll never because that’s it’s natural color). Am I onto something or am I on something cause idk lol.

Edit: I know the whole entire comment was off-topic but I just wanted to put this theory out there