There are many diseases that cause melanosis on the face and the cause varies from disease to disease. For example.
Nevus of Ota and coffee spots are both birthmarks.
melasma, on the other hand, is the result of a combination of endocrine, sun exposure and emotional factors.
the use of certain medications, especially photosensitive ones, which tend to cause hyperpigmentation of the face
certain inflammatory skin conditions that tend to occur on the face can leave hyperpigmentation after they have subsided
Intense sun exposure and inappropriate cosmetics can cause facial hyperpigmentation.