5 month old choco lab tail graying

Question
I noticed my lab's tail turning white at the roots. I see him nip at his tail once in a while.  I bathe him with Vet recommended shampoo Dermolyse 2x a week.  The vet said he might be low in vitamins and prescribed folic acid supplement.  I feed him Optima 27.  What could be causing the discoloration? Thanks for the help.

Answer
You might get a second opinion.  I really question having to give vitamins in addition to a premium dog chow.  I don't remember seeing that in dogs eating a common brand.  

Feed a decent chow and as little else as possible, seldom if ever bathe the dog, and brush it daily with a soft bristled brush.  You might add a teaspoon a day of liquid cooking oil to its food, but most dogs don't need it.