It is normal to have hyperpigmentation
on the buttocks,in between the butts,in between the thighs.This is mainly caused by constant friction over the skin either by sitting,walking,running....etc.This can look uncomfortable while wearing swimsuits during summer.
If you want to lighten the skin tone over the region,I suggest you follow the following regimen:
1.Once in a week on saturday soak yourself in bathtub filled with warm water mixed with body wash.Soak yourself for atleast 1 hrs and then take a regular bath
2.On sunday while taking bath you can scrub your butt with walnut scrub or besan mixed with turmeric,lemon and curds.This acts a natural bleaching agent and removes the dead skin and lighten the skin tone.
3.The other option is you can go to cosmetology clinic where they will either bleach the area or do glycolic acid
peel 1 or 2 sittings to lighten the area.
4.You can regularly use dermadew soap and dermadew lotion to maintain the moisture of the skin over your body.
Hope it helps.