Melanin is the brownish pigment responsible for the color of skin, hair, and the iris of the eyes. It is produced by melanocytes. Most people have similar numbers of melanocytes, and the wide range of color shades of human skin is due to the amount of melanin that is produced rather than the number of melanocytes. There are different subtypes of melanin, the main ones in the skin being
Eumelanin (brown and black types)
Pheomelanin, which has a reddish hue
Ultraviolet radiation, as in sunlight, stimulates melanin production, as do a number of pathologic processes. Other factors can interfere with melanin production.
Pigmentation disorders involve hypopigmentation, depigmentation, or hyperpigmentation. Areas may be focal or diffuse.
In hypopigmentation, pigment is decreased, whereas in depigmentation, pigment is completely lost, leaving white skin.
Focal hypopigmentation is most commonly a consequence of
Injury
Inflammatory dermatoses (eg, atopic dermatitis Atopic Dermatitis (Eczema) Atopic dermatitis is a chronic relapsing inflammatory skin disorder with a complex pathogenesis involving genetic susceptibility, immunologic and epidermal barrier dysfunction, and environmental... read more
, psoriasis Psoriasis Psoriasis is an inflammatory disease that manifests most commonly as well-circumscribed, erythematous papules and plaques covered with silvery scales. Multiple factors contribute, including... read more 
)Chemical exposure (especially to hydroquinones and phenols)
Focal hypopigmentation or depigmentation is also a feature of vitiligo Vitiligo Vitiligo is a loss of skin melanocytes that causes areas of skin depigmentation of varying sizes. Cause is unknown, but genetic and autoimmune factors are likely. Diagnosis is usually clear... read more 
(which may involve large areas of skin), leprosy Leprosy Leprosy is a chronic infection usually caused by the acid-fast bacilli Mycobacterium leprae or the closely related organism M. lepromatosis. These organisms have a unique tropism... read more 
, nutritional deficiencies (kwashiorkor Primary PEU ), genetic conditions (eg, tuberous sclerosis Tuberous Sclerosis Complex (TSC) Tuberous sclerosis complex is a dominantly inherited genetic disorder in which tumors (usually hamartomas) develop in multiple organs. Diagnosis requires specific clinical criteria and imaging... read more 
, piebaldism, Waardenburg syndrome), morphea (localized scleroderma, in which skin is usually sclerotic), lichen sclerosus Lichen Sclerosus Lichen sclerosus is an inflammatory dermatosis of unknown cause, possibly autoimmune, that usually affects the anogenital area. Diagnosis is usually clinical and supported by skin biopsy. Treatment... read more 
, pityriasis versicolor (or tinea versicolor Tinea Versicolor Tinea versicolor is skin infection with Malassezia furfur that manifests as multiple asymptomatic scaly patches varying in color from white to tan to brown to pink. Diagnosis is based... read more 
), idiopathic guttate hypomelanosis, progressive macular hypomelanosis, postinflammatory hypopigmentation, and pityriasis alba.
Diffuse hypopigmentation is most often caused by
Hyperpigmentation Hyperpigmentation Hyperpigmentation has multiple causes and may be focal or diffuse. Most cases are due to an increase in melanin production and deposition. (See also Overview of Pigmentation Disorders.) Focal... read more 
typically occurs after inflammation resulting from various causes. This postinflammatory hyperpigmentation is usually focal in distribution. Hyperpigmentation may also be caused by a systemic disorder, drug, or cancer; in such cases, distribution is usually more diffuse.
