Hair color is the pigmentation of hair follicles due to the two types of melanin, eumelanin and phaeomelanin. Generally, if more melanin is present in the hair, the color of the hair is darker; if less melanin is present, the hair color is lighter. A person's hair color may change over time due to the varying levels of melanin, and some hair can have follicles of more than one color. Due to migrations and global travel, considerable differences exist in the hair colors of individuals, even between individuals of similar ethnicity, creating a greatly increased diversity of hair color.
Phaeomelanin is more chemically stable than black eumelanin, but less chemically stable than brown eumelanin, so it breaks down more slowly when oxidized. This is the reason bleach will cause darker hair to turn reddish-brown during the artificial coloring process. As the phaeomelanin continues to break down, the hair will gradually become orange, later yellow, and then white.
The genetics of hair colors are not yet firmly established. According to one theory, at least two gene pairs control human hair color. One gene, which is a brown/blonde pair, has a dominant brown allele and a recessive blonde allele. A person with a brown allele will have brown hair; a person with no brown alleles will be blonde. This also explains why two brown-haired parents can produce a blonde-haired child. The other gene pair is a not-red/red pair, where the not-red allele (which suppresses production of pheomelanin) is dominant and the allele for red hair is recessive. Since the two gene pairs both govern hair color, a person with two copies of the red-haired allele will have red hair, but it will be either auburn or bright reddish orange depending upon whether the first gene pair gives brown or blonde hair, respectively.
The two-gene model does not account for all possible shades of brown, blonde, or red (for example, platinum blonde versus dark blonde/light brown), nor does it explain why hair color sometimes darkens as a person ages. Several gene pairs control the light versus dark hair color in a cumulative effect. A person's genotype for a multifactorial trait can interact with environment to produce varying phenotypes (see quantitative trait locus).
Blond hair ranges from nearly white (platinum blond, tow-haired) to a dark golden blond. Strawberry blond, a mixture of blond and red hair found predominantly in Ireland, is an especially rare type containing the most phaeomelanin. Blond hair can have almost any proportions of phaeomelanin and eumelanin, but both only in small amounts. More phaeomelanin creates a more golden blond color, and more eumelanin creates an ash blond. Blond hair is common in many European peoples, but rare among others. Many children born with blonde hair may develop darker hair as they age.
Red hair ranges from vivid strawberry shades to deep auburn and burgundy, and is the rarest hair color on earth. It is caused by a variation in the Mc1r gene and believed to be recessive. Red hair has the highest amounts of phaeomelanin and usually low levels of eumelanin.
A change in hair color typically occurs naturally as people age, usually turning their hair to gray and then white. More than 40 percent of Americans have some gray hair by age 40, but white hairs can appear as early as childhood. The age at which graying begins seems to be almost entirely based on genetics. Sometimes people are born with gray hair because they inherit the trait genetically.
Two genes appear to be responsible for the process of graying, Bcl2 and Bcl-w. The change in hair color is caused when melanin ceases to be produced in the hair root and new hairs grow in without pigment. The stem cells at the base of hair follicles are responsible for producing melanocytes, the cells that produce and store pigment in hair and skin. The death of the melanocyte stem cells causes the onset of graying.
Malnutrition is also known to cause hair to become lighter, thinner, and more brittle. Dark hair may thus turn reddish or blondish due to the decreased production of melanin. The condition is reversible with proper nutrition.
A recent study demonstrated that people 50-70 years of age with dark eyebrows but gray hair are significantly more likely to have type II diabetes than those with both gray eyebrows and hair.
Gray hair may temporarily darken after inflammatory processes, after electron-beam-induced alopecia, and after some chemotherapy regimens. Much remains to be learned about the physiology of human graying.
There are no special diets, nutritional supplements, vitamins, nor proteins that have been proven to slow, stop, or in any way affect the graying process, although many have been marketed over the years. This may change in the near future. French scientists treating leukemia patients with a new cancer drug noted an unexpected side effect: some of the patients' hair color was restored to their pre-gray color.
The process of changing a person's hair color can be done by a chemical process known as hair coloring. Hair coloring can be permanent or temporary and the lasting effects are determined, in part, by the texture of the individual's hair.
The use of chemical lighteners, such as bleach, is one way hair is lightened or "highlighted". This type of hair coloring is always permanent because it involves the removal of natural pigment, which never returns. Semi-permanent hair color can darken or change the tonal value of the hair, but cannot lighten the hair and can usually be completely washed away after several shampoos. Semi-permanent hair color is only a deposit of hair color. This hair color is used to darken natural hair color. "Rinses" are a form of temporary hair color that are usually applied to hair during a shampoo. Their effects usually only last until the hair is shampooed or rinsed. Permanent hair color is probably the most-utilized because of its ability to affect the hair in level (lightness or darkness) as well as tone, but it comes with a unique set of potential problems, such as the need to frequently re-apply, unwanted fading and hot roots.