What causes darkening of the skin?

This increased melanin is what gives skin of color its warm shade. One can produce more melanin if stimulated by the sun, medications, or certain diseases.
The most obvious example of this is tanning, which occurs when our skin produces more melanin after sun exposure. Our skin may also darken in response to certain drugs such as minocycline, or in response to certain medical conditions such as Addison’s disease.