TLDR Hair color is determined by different melanins and changes with age.
The document reviewed research on hair pigmentation, highlighting that hair color is determined by the presence or absence of different melanins, unlike skin color which also involves hemoglobins and carotenoids. The physical properties of the hair shaft only slightly modify hair color. Evolutionary pressures have led to a wide range of natural hair colors among different ethnic groups, from yellow to black, and gray/white hair signifies aging. Melanins in hair are produced in melanosomes by melanocytes through melanogenesis, a complex biochemical pathway. The review summarized findings from the 4th Intercontinental Meeting of Hair Research Societies in 2004.
450 citations,
January 2005 in “The journal of investigative dermatology/Journal of investigative dermatology” Hair color is determined by melanin produced and transferred in hair follicles.
65 citations,
June 2003 in “EMBO journal” Noggin overexpression delays eyelid opening by affecting cell death and skin cell development.
86 citations,
August 2000 in “Pigment cell research” Melanocyte activity in hair follicles is linked to the hair growth cycle, being active in growth phases and inactive in rest phases.
21 citations,
May 2022 in “Frontiers in Cell and Developmental Biology” Hair growth and health are influenced by factors like age, environment, and nutrition, and are controlled by various molecular pathways. Red light can promote hair growth, and understanding these processes can help treat hair-related diseases.
21 citations,
May 2016 in “Experimental and Therapeutic Medicine” 
224 citations,
March 2006 in “Seminars in Cutaneous Medicine and Surgery” The document concludes that understanding hair follicle biology can lead to better hair loss treatments.
September 2004 in “Experimental dermatology” Melatonin directly affects mouse hair follicles and may influence hair growth.
