chocolate lab tail is turning white

Question
My 6 month chocolate lab's tail is turning white. I have also discoverd traces of white hair on her back. I feed her premium food and as far as I know, she is a purebred. Is this normal and will her coat ever turn brown again?

Answer
No it isn't normal.  Occasionally pure bred Labs are born with marking, but they are not something that develops at 6 months.  

My gut feeling it is the food.  You can trust the common brands, such as Pro Plan, Iams, and even Science; but the premium brands just don't have the proven track record they do.