Excess skin pigmentation or hyper-pigmentation is usually caused by an overwhelming of the melanin-protective system. This leads to excess production and clumping of melanin, and the appearance of brown or darker spots of pigmentation in the skin.
There are various causes of hyper-pigmentation both external and internal factors. The commonest external cause of is exposure to ultra violet radiation from the sun whereas the commonest internal cause is hormonal changes.
The skin’s natural protection mechanism is to inject melanin or pigment like an umbrella over the cell nucleus, to try to prevent DNA damage. Unfortunately, too much UV oxidises the melanin, creating the freckles or dark spots, and will still damage skin cell DNA. Once the DNA has mutated, it will keep repeating this cell mutation.