Hyperpigmentation is caused by an increase in melanin, the substance in the body that is responsible for color (pigment). It causes the skin to appear dark than it was previously. Exposure to sunlight is a major cause of hyperpigmentaion, and will darken already hyperpigmented areas. Hyperpigmentation can also be caused by various drugs, including some antibiotics, antiarrhythmics and antimalarial medication.
Hypopigmentation is the result of a reduction in melanin production. It causes the skin to appear lighter than it was previously. Examples of hypopigmentation include:
Vitiligo: an auto immunue disorder that causes white patches of skin all over the body
Albinism: an inherited disorder caused by the absence of melanin. This results in a complete lack of pigmentation in skin, hair or eyes.
Pigmentation loss as a result of skin damage