The term pigmentation refers to the ranges of colour within the skin. Pigmentation can range from simple freckles, age spots and sun spots to large areas of discolouration like in Melasma. There are many causes and types of pigmentation which can be caused by hormones, sun damage, trauma and genetics.
There are special cells called melanocytes in the skin that make melanin, a pigment that controls the colour of our skin. The actual number of melanocytes within our skin is the same regardless of skin type, it is the production of melanin from these cells which determines skin colour. High levels of melanin cause darker skin and lower levels cause lighter skin.
Hypopigmentation refers to areas of lightened skin and hyperpigmentation refers to areas of darkened skin, both are caused by altered levels of melanin. This occurs when the melanocytes are damaged or not functioning adequately, therefore affecting melanin production (either stopping production or increasing production).
Triggers for melanin production such as the sun, heat, injury and hormones cause the melanin to be produced in the deep layers of the skin. The melanin then slowly rises through the layers to the skin to sometimes sit more superficially (such as freckles and sun spots).
This is why some pigmentation is difficult to see and difficult to treat as it sits deep within the dermis of the skin. Melasma is a condition where the melanin can sit both superficial and deep making it much more difficult to treat.