Dark purple skin can be caused by heredity, environment, melanosis, acanthosis nigricans, hypoadrenocorticism and so on. 1. Heredity: Skin color is related to hereditary factors, and some people are born with darker skin color. In addition, skin color is related to ethnicity, for example, people in cross-equatorial regions generally have darker and purple skin color. 2. Environment: prolonged exposure to sunlight leads to excessive exposure to ultraviolet rays, in addition, with the increase in age, easy to appear pigmentation, so the skin will be dark purple. 3. Melanosis: The cause of the disease is not known, but it may be related to frequent contact with photosensitive substances, the use of low-quality cosmetics, long-term exposure to the sun and other factors, a large number of pigmentation of the skin mucous membranes, resulting in dark purple skin. 4. Acanthosis nigricans: it may be related to heredity, obesity, autoimmune abnormality and other factors, mostly manifested as deepening of skin color, papillary or velvety thickening of the skin, which is easy to appear in the neck, armpits and other skin folds. 5. Hypoadrenocorticism: the adrenal cortex synthesizes and secretes fewer glucocorticoids and hydrocorticoids, resulting in a characteristic deepening of the skin color all over the body, which is mostly located in the exposed, friction areas and areolas. Dark purple skin may be related to a variety of factors, it is recommended to consult a doctor to clarify the diagnosis.