Dark skin on the buttocks is not as unusual you may think it is. Skin color in areas such as the thighs, buttocks, elbows, knees, and underarms is often darker than the skin color of the rest of the body, particularly in people who have darker complexions. This is at least partly due to the friction these areas experience. These parts of the body are always in contact with clothes or with other surfaces, and the constant friction makes the skin dark and rough. To a large extent, the situation is unavoidable. Some people's skin simply is more affected by this friction, and it is not possible to avoid it entirely.
However, you can try using different, gentler materials for your underwear as well as your clothes. After a few months, there should be some noticeable difference. To lighten the skin, you can try applying a paste of cucumber juice with sandalwood powder and pinch of turmeric for about half an hour a day. You can also apply raw potato juice or aloe gel. For the roughness, you can try exfoliating. You can either use a readymade exfoliating products or create your own mixture using baking soda and water, or whole wheat flour and milk.
answered by G M