Hair color is the result of pigmentation due to the presence of the chemicals eumelanin and pheomelanin. In general, the more melanin present, the darker the hair color; the less melanin, the lighter the hair color. Black hair predominates outside of Europe. A person's hair color may also change over time and may be more than one color at a time. Considerable differences in color and texture exist between individuals of similar ethnicity, and immigration and global travel have greatly increased the diversity of hair characteristics among many countries.
Natural hair colors
Natural hair color is black, brown, blond, or red, depending on the ethnic origins of the person in question. Black hair is the most common. Hair color is genetically associated with certain skin tones and eye colors. Blond and red hair are sometimes associated with disorders such as skin cancer or albinism. 
Black hair is the darkest and most common color of human hair. Black hair is most commonly found in people of non-European heritage, but occurs in people of all backgrounds and ethnicities. It has large amounts of eumelanin and is less dense than other hair colors. It can be a very dark brown or complete black. Black hair is known to be the shiniest out of all other hair colors.
Brown hair is the second most common hair color (after black hair) and is also found all over the world (mostly European, Middle Eastern, African, or some Native American heritage). 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. Brown-haired people have medium-thick strands of hair. Brown haired people can range highly in skin and eye color. A brown-haired male may be called a brunet, though this usage is relatively uncommon; a brown-haired female may be called a brunette.
Blonde hair is a relatively rare human phenotype due to its association with recessive genes, occurring in approximately 2% of the European population.
Blonde hair ranges from nearly white (platinum blonde, tow-haired--though this shade is extremely rare) to a dark golden blonde. Strawberry blonde (found predominantly in the British Isles) is an especially rare type (the most pheomelanin): a mixture of blonde and red hair. Blonde hair can have almost any proportions of pheomelanin and eumelanin, but both only in small amounts. More pheomelanin creates a more golden blonde color, and more eumelanin creates an ash blonde. The most pheomelanin creates a strawberry blonde. Natural blondes have the thinnest strand of hair. Males with this hair color are blond; females are blonde.
Red hair is the least common hair color. It ranges from vivid strawberry shades to deep auburn and burgundy. Celtic heritage is common with red hair. Red hair is caused by a mutation of the Mc1r gene and believed to be recessive. Red hair has the highest amounts of pheomelanin and usually low levels of eumelanin.
Gray hair color typically occurs naturally as people age. In some cases, gray hair may instead be caused by a deficiency of B12 or a thyroid imbalance.