Greetings; it may be common for some people to have darker shade of the colour of the skin of the buttocks. However, this could alos have occured dut to past allergies of fungal infections. you could use glycolic acid based creams daily night - avoid sensitive areas - and this should improve the colour. see a qualfied deramtologsit - rule out any existing infection, esp if you have any itching or other symptoms. Warm regards, Dr Govind Suresh Mittal.