It's possible her shell has been stained by something - maybe bacteria, maybe some sort of dye - and eventually the unusual color will shed off. When she's sees the vet, ask the vet's opinion.
And like someone said in an earlier post, turtle shells are only green when they are young - green with yellow patterns. When they grow up, their carapaces turn either brown or olive drab or even black. The brown color is very pretty - sort of chestnut with pale stripes deep inside. Your turtle's carapace will be brown when he's cleaned up. The underneath side, including the underside of the marginal scutes, is typically yellow with black spots. The yellows will deepen to a lovely yellow orange if you feed them foods containing beta carotene - like carrots.
