Human hair color
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.

# Genetics and biochemistry of hair color
There are two types of pigments that 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 in the hair results in blonde hair, whereas a higher concentration of brown eumelanin will color the hair brown. High amounts of black eumelanin results in black hair, while low concentrations of black eumelanin in the hair results in gray hair color. Although most noticeable in red-heads, all humans have varying concentrations of 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 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).

# Natural hair colors
Natural hair color is black, brown, blonde, or red, depending on a person's ethnic origins. Hair color is genetically associated with certain skin tones and eye colors.
- Black hair
Black hair
- Dark brown hair
Dark brown hair
- Brown hair
Brown hair
- Auburn hair
Auburn hair
- Red hair
Red hair
- Blonde hair
Blonde hair
- Grey (gray) hair
Grey (gray) hair
- White hair
White hair

## Black hair
Black hair is the darkest and most common color of human hair. It has large amounts of eumelanin and is less dense than other hair colors. 

## Brown hair
Brown hair is the second most common hair color, the most common in Europe. It is characterized by higher levels of the dark pigment eumelanin and lower levels of the pale pigment phaeomelanin. Of the two types of eumelanin (black and brown), brown-haired people have brown eumelanin; they also usually have medium-thick strands of hair. Brown-haired people are also known as brunettes.

## Blonde hair
Blonde hair ranges from nearly white (platinum blonde, tow-haired) to a dark golden blonde. Strawberry blonde, a mixture of blonde and red hair - thought to have originated in Celtic and Scandinavian countries - is a much rarer type containing the most amounts of phaeomelanin.
Blonde hair can have almost any proportions of phaeomelanin and eumelanin, but both only in small amounts.  More phaeomelanin creates a more golden blonde color, and more eumelanin creates an ash blonde. Blonde hair is common in many European peoples, but rare among peoples of non-European origins. Many children born with blonde hair develop darker hair as they age.

## Auburn hair
Auburn hair ranges from light to reddish brown. The chemicals which cause auburn hair are eumelanin (brown) and phaeomelanin. It is most commonly found in individuals of European descent.

## Red hair
Red hair ranges from vivid strawberry shades to deep auburn and burgundy, and is the rarest fully distinct 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.

## Grey and white hair
Grey hair color typically occurs naturally as people age (see "Effects of aging on hair color", below). For some people this can happen at a very young age (ie: at the age of 10).  The same can be said for white hair. In some cases, grey hair may instead be caused by thyroid deficiencies or a deficiency of B12.

# Conditions affecting hair color

## Effects of aging on hair color
Children born with a certain color may find that it gradually darkens as they grow. Many blonde, strawberry blond, light brown, or red haired infants experience this.
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.

## Other medical conditions affecting hair color
Albinism is a genetic abnormality in which little pigment is found in human hair, eyes or skin.  This results in gray, blue, or purple eyes.  The skin is pale and the hair is white or pale blond.
Vitiligo is a patchy loss of hair and skin color that may occur as the result of an auto-immune disease.
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.
Werner syndrome and pernicious anemia can also cause premature graying.
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.

## Artificial factors affecting hair color
A 1996 British Medical Journal study conducted by J.G. Mosley, MD found that tobacco smoking may cause premature graying. Smokers were found to be four times more likely to begin graying prematurely, compared to nonsmokers in the study.
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.

## Changes in hair color after death
The hair color of mummies or buried bodies can change over large time periods. Hair contains a mixture of black-brown-yellow eumelanin and red pheomelanin. Eumelanin is less chemically stable than pheomelanin and breaks down faster when oxidized. It is for this reason that Egyptian mummies have reddish hair.  The color of hair changes faster under extreme conditions. It changes more slowly under dry oxidizing conditions (such as in burials in sand or in ice) than under wet reducing conditions (such as burials in wood or plaster coffins).

# Hair coloring
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.