The colour of the skin is mainly due to the amount of brown melanin pigment (The pigment that gives human skin its color) mixed with blue (from reduced haemoglobin), red (from oxyhaemoglobin) and yellow (from carotenoids in the diet). The amount of melanin is determined by constitutional colour (white, brown or black skin) and skin phototype, i.e. the result of exposure to ultraviolet radiation (tanning).
Increase in melanin (hyperpigmentation or hypermelanosis) can be due to an increased number of pigment cells (melanocytes) or from increased production of melanin.
Increased melanin production, also known as hyperpigmentation, can be a few different phenomena:
Melasma describes the darkening of the skin.
Chloasma describes skin discolorations caused by hormones. These hormonal changes are usually the result of pregnancy, birth control pills or estrogen replacement therapy.
Freckles are small flat brown marks arising on the face and other sun exposed areas. They are most often seen in fair skinned people, especially those with red hair, but they are an inherited characteristic that sometimes affects darker skin types as well.
Solar lentigo, also known as "liver spots" or "senile freckles" refers to darkened spots on the skin caused by aging and the sun. These spots are quite common in adults with a long history of unprotected sun exposure. On lighter to medium skin tones, solar lentigenes emerge as small- to medium-sized brown patches of freckling that can grow and accumulate over time on areas of the body that receive the most unprotected sun exposure, such as the back of the hands, forearms, chest, and face.
Aside from sun exposure and hormones, hyperpigmentation can be caused by skin damage, such as remnants of blemishes, wounds or rashes. This is especially true for those with darker skin tones. The most typical cause of darkened areas of skin, brown spots or areas of discoloration is unprotected sun exposure.