Melanin is responsible for our skin colour, but when overproduced, it results in hyperpigmentation.
Other causes include:
• Excessive UV or sun exposure
• Hormonal changes
• Trauma to the skin such as cuts, burns and acne among others
• Birthmarks which can appear in childhood and adulthood
• Medications and medical conditions such as liver disease
Some causes are common, but there are those, which are less common like haemochromatosis which is a genetic disease.