Every natural hair color on Earth is created by just two pigments: eumelanin and pheomelanin.

The biology of hair color is centered in the hair follicle, where specialized cells called melanocytes produce pigment. These cells synthesize two types of melanin: eumelanin, which is chemically structured as a cross-linked polymer of 5,6-dihydroxyindole, and pheomelanin, which contains sulfur-containing benzothiazine units. The MC1R gene on chromosome 16 acts as a genetic switch that determines which pigment is produced. When the MC1R receptor is activated, melanocytes produce eumelanin, but if the receptor is blocked or mutated, they produce pheomelanin instead.A study published in 'Nature Communications' in 2016 identified the IRF4 gene as a key regulator of graying, which relates to the depletion of melanocyte stem cells. Dark hair contains a high concentration of eumelanin, while blond hair results from a very low concentration of both pigments. Red hair is unique because it contains high levels of pheomelanin and very little eumelanin, a trait often linked to specific mutations in the MC1R gene found in about 1-2 percent of the global population.As humans age, the enzyme tyrosinase, which is essential for melanin production, becomes less active. Additionally, research from the Federation of American Societies for Experimental Biology (FASEB) in 2009 showed that hair turns gray due to a massive build-up of hydrogen peroxide in the hair follicle. This oxidative stress wears down the pigment from the inside out because the body produces less of the neutralizing enzyme catalase. Eventually, the melanocytes cease to function entirely, leaving the hair shaft transparent, which we perceive as white.