Hair color is the pigmentation of hair follicles due to two types of melanin, eumelanin and phaeomelanin. Generally, if more melanin is present, the color of the hair is darker; if less melanin is present, the hair is lighter. Levels of melanin can vary over time causing a person's hair color to change, and it is possible to have hair follicles of more than one color.

Particular hair colors can be associated with ethnic groups - however, due to migration and global travel, considerable variations have developed in the hair color of individuals within an ethnic group, creating a greatly increased diversity of hair color.

Genetics and biochemistry of hair color

Two types of pigments give hair its color: eumelanin and phaeomelanin. Phaeomelanin colors hair red. Eumelanin, which has two subtypes of black or brown, determines the darkness of the hair color. A low concentration of brown eumelanin results in blonde hair, whereas a higher concentration of brown eumelanin will color the hair brown. High amounts of black eumelanin result in black hair, while low concentrations give gray hair. Although most noticeable in redheads, all humans have some phaeomelanin in their hair.

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 why bleach gives darker hair a reddish tinge during the artificial coloring process. As the phaeomelanin continues to break down, the hair will gradually become orange, then yellow, and finally 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, (brown/blonde) 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 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. 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 on 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).

Natural hair colors