Hair grows out gray.
More Info: Human hair grows out gray from the roots due to an absence or inactivity of melanocytes.
As with pigmentation in the rest of the body, such as skin color, hair color is made up of a pigment called melanin, which is formed before birth. Pigment cells, called melanocytes, are located at the openings of hair follicles where they then color hair by injecting melanin into keratin cells, the same cells that make up skin and nails. However, as you grow older, the melanocyte activity around the hair follicles slows down or stops, which then in turn reduces the amount of pigment the hair acquires. The reduction of melanocyte activity occurs right before the hair sheds, hence-why hair looks gray around the roots.
What Makes Gray Hair Turn Gray?
Scientists also attribute the graying of hair to both intrinsic and extrinsic factors. Intrinsic factors include hormones, age, and genetic defects. Although age is the most telling reason for gray hair, the “biological clock” on some will halt melanocyte activity sooner than others. Extrinsic factors include chemical exposure, pollutants, climate or toxins which will subsequently slow down the production of melanin.
“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
“Hair Foundation Article – Why Does Hair Turn Gray and White?.” Welcome to Hair Foundation. N.p., n.d. Web. 10 Jan. 2013. http://hairfoundation.org/hair-library/article-gray-hair.htm