A lack of pigment is what makes your hair to turn gray.
More Info: Hair is naturally colorless, but there are cells called melanocytes in your hair follicles, and these cells are what give your hair its color. As you age, these cells die and/or become less active and therefore the hair cells receive less pigment, causing them to be gray or white. Aging and genetics are the most common causes of gray hair, but other factors can play a part.
What Causes Gray Hair
Genetics help determine the age at which you will start to go gray. Generally, you will start to go gray around the same time as your parents did, although your hair may gray faster or slower than theirs did. Also, factors such as disease, pollution, and malnutrition can affect this process. Children and teens who are malnourished, or who suffer from anemia, thyroid conditions, or a vitamin B12 deficiency, for example, will see gray or white hair develop. In cases like these, though, once the disease or condition has been treated, the hair will usually return to its normal color. Smoking can also lead to premature gray hair, as well as extreme or prolonged stress and exposure to certain chemicals and toxins.
“Why does hair turn gray? Everyday Mysteries:Fun Science Facts from theLibrary of Congress).” Library of Congress Home. N.p., n.d. Web. 10 Jan. 2013. http://www.loc.gov/rr/scitech/mysteries/grayhair.html
Feature, Daphne SashinWebMD. “Premature Graying: Reasons, Options.” WebMD. WebMD, 18 Feb. 0004. Web. 10 Jan. 2013. <http://www.webmd.com/healthy-beauty/guide/abcs-premature-graying>.