Welcome to BYC.
What's the soil like in your area? A chicken that's been dustbathing in certain soils can pick up some staining, often from the iron content or from the organic material in it.
Sometimes white chickens will yellow from sun exposure -- though I'd expect that more on a mature hen rather than a young pullet.
Sometimes white chickens are off-colored genetically.
BUT, when I blow up your photos and look closely at the feathers, the discoloration is spotty and looks like dirt stuck to the feathers. This makes me suspect that she was kept in crowded, damp, or less-than-clean conditions before she had the good luck to come to you. Though I will say that I had a Delaware I named "Pigpen" after the Charlie Brown character, because while all the other white birds in the flock were beautiful she was ALWAYS getting dirty -- I think she perched underneath the others just close enough to get pooped on at night, so sometimes it's just the bird and not the conditions.