Changes in skin colour are common and are usually harmless. Skin colour is determined by melanin, a protein produced by cells called melanocytes. Darker skin contains a higher concentration of melanin, while lighter skin contains a lower concentration. Your melanin level is genetically determined, but sun exposure also plays an important part. As the skin tries to protect itself from the sun's harmful ultraviolet rays, melanin levels increase, causing the skin to darken.

Hyperpigmentation
Increased pigmentation can cause certain areas of the skin to darken.

Melasma is a condition in which some areas of skin on the cheeks, forehead and upper lip take on a dark colour. It may be caused by hormonal changes during pregnancy or the menopause, use of the oral contraceptive pill, or hormone replacement therapy. When hormones settle, melasma usually fades. Liver spots, also known as age spots, are dark patches that usually appear on the hands and other sun-exposed areas of the body. They increase as ageing skin becomes more delicate and sensitive to the sun's ultraviolet rays. Areas of skin exposed to perfume or scented cosmetic products together with sunlight may also darken temporarily.

These conditions may be annoying, but they are not harmful. For other forms of hyperpigmentation you may need to see your doctor. Darkening of the skin without sun exposure may be a sign of another disease, such as Addison's disease or Cushing's disease. Moles may also form or change as a result of sun exposure.

Hypopigmentation
Vitiligo causes certain areas of the skin to lose colour. In vitiligo, some areas of skin stop producing melanin. As a result, symmetrical white patches form, usually on the face, hands, armpits and groin. In some people these patches start producing pigment again without treatment. The cause of vitiligo is unknown, but it may be an autoimmune disorder affecting the melanocytes, the pigment-producing cells.

Albinism is a rare inherited disease in which melanin, which gives colour to the skin, hair and eyes, is absent from birth. People with albinism have extremely fair skin and hair and are extremely sensitive to sunlight. Phenylketonuria is a genetic disorder that causes low melanin levels and very fair, though not white, skin and hair.

Psoriasis and other skin conditions that cause scaling can lead to a lack of melanin in the skin after the treatment used to clear the scaling. Tinea versicolor can also cause a similar loss of pigment.

Treatment
When psoriasis or tinea versicolor is treated effectively, normal pigmentation gradually returns. Depigmenting creams can help lighten dark areas in other conditions. However, because these creams also lighten normal skin, they should be applied carefully.

You can also cover patches caused by vitiligo, or age spots, with a cosmetic product. For vitiligo, your doctor may recommend psoralen, a medicine that makes the skin more sensitive to light and ultraviolet rays (together called PUVA). Tretinoin cream may also help lighten skin patches caused by melasma. Laser treatments are also available to lighten dark skin patches.