The new hair should grow through white, and so over time the yellowing will decrease?
I used to bath Storm's feathering on her back legs every week when she was young (before she learnt to keep herself clean). I also used baby wipes to clean the area after she had been outside and that was enough to keep her white.
You can get shampoos specifically for 'white' coats...have never used one, but I dont know whether that would help with the staining?