From your presentation, it looks like either you have excessive xerosis over the gluteal region or you might be having fungal infection
infection which is healing with hyperpigmentation
See as far as Xerosis( excessive dryness) is concerned,, keep the area adequately moisturized...apply plenty of moisturizing oils/lotions/ creams over the affected area daily atleast twice a day....
But if you feel that there is redness
or itching along with pigmentation, then it might be a fungal infection for which you might need to see a dermatologist
for proper treatment...
But whatever be the case, i would advise you to avoid touching the area or itching as it tends to lead to pigmentation.
Hope it helps
Dr Geetika Paul