Hi, I can understand your concern for the problem of discoloration in butt crack after yeast infection. It is known as post- inflammatory hyper-pigmentation. This is due to activity of melanocytes which causes deep pigmentation on aforesaid areas. Melanocytes are the color producing cells of skin. This is a common sequel to all type of skin infections and varies form person to person accordingly. The most important part in treatment is by lightening agents such as Kojic acid or azelaic acid cream application which you can use under dermatological guidance. Eat healthy diet containing lot of Vitamin A and Vitamin C. Do not try applying over the counter products as these can further aggravate the situation. Prevent yourself form heat and humidity as are the predisposing factor for the recurrence of yeast infection. Take care.