"Pantothenic Acid and folic acid taken together help reverse graying"

This was interesting.  

White hair has no melanin left at all leaving only the colour of the keratin, the protein that makes up hair, showing through. 

And this . . . 

Premature graying occurs in younger people even children. The reason for early gray hair is uncertain, but factors such as illness or autoimmune problems may be the cause. 

The second article I link to points to abdominal or intestinal surgeries as another cause of premature graying and whiting.  

Regardless of how young we are when our hair starts to turn gray or white, it gets harder and harder for us to think that we look like George Clooney or that distinguished gentleman who claims to be the most interesting man in the world.  We just look older, particularly in the eyes of child-bearing females.  So is there a way to reverse white hair and restore color to it naturally through nutritional compounds?  Maybe. 

Conditions such as illness or disorders of the autoimmune system affect hair growth and sometimes the color. Conditions such as thyroid disorders, a deficiency in vitamin B12 found in meat, eggs, and milk are linked with graying sooner than the average person.

Vitiligo, a skin condition that causes white patches on otherwise healthy skin due to a loss of melanocytes, may be another culprit in graying hair. 

Abdominal and intestinal surgeries such as gastric by-pass may contribute to premature graying.  Early menopause and smoking are two more reasons to look into if gray hair has become evident. 

Yeah, that last part about abdominal and intestinal surgeries makes sense to me.  Once you start disrupting the digestive tract with all of the important bacteria, something is not going to work well.  The same article that pointed to abdominal and intestinal surgeries and complications as a source for graying and whiting of one's hair, suggested the following nutritional compounds that MAY combat aging hair and even reverse color back to original or darker shade.  

Vitamin B-6 and B-12 are two of the Complex-B vitamins that aid in healthy skin and hair. B-6 may help restore hair to its original color following an illness or deficiency.

Para-Amino benzoic Acid (PABA) and Pantothenic Acid are part of the family of B-complex vitamins.  Both of these vitamins are available at health food stores and pharmacies.  Use them to postpone the onset of graying. It is purported that when Pantothenic Acid and folic acid are taken together it helps reverse graying by taking the hair back to its original color.

Inositol, a substance found naturally in the human body, may encourage the growth and overall health of hair.

Biotin produces keratin which is a major component of hair and nails.

Niacin contributes scalp circulation which in turn nourishes the hair follicles. 

Okay, the point about Niacin may be true, but does the circulation alone produce darker hair follicles?  I have my doubts.  Nor does the article estimate how long one must be on Niacin to get a kind of hair color restoration.  And PABA and B5 "postpone the onset of graying."  Based on what evidence?  

Some claim that copper deficiency is the main culprit for premature graying.  

Because copper plays an important role in melanin formation, a lack of copper can affect a person’s hair. Melanin is important for pigmentation (color) of a person’s hair. Therefore, some doctors theorize that low copper levels could affect a person’s hair color, potentially leading to premature graying hair. 

First, it should be known that one needs very little copper.  Copper competes with zinc for absorption, so if you're supplementing with copper, you only need 2mcg.  Second, supplementing with copper carries its own risks, like oxidizing B vitamins, organs, and tissue.  Not good.  In fact, most of us are probably NOT deficient in copper.