What is the best way to lighten skin on the butt and thighs? (photo)

Luminase and Exfolase creams are recommended for dark discoloration on the skin. The product works well on Indian skin. I would combine with fractional laser. 
Raffy Karamanoukian MD FACS

East Asian and Indian skin is prone to hyperpigmentation.
Chronic moisture can cause skin irritation and result in hyperpigmentation of the inner thighs , posterior thighs and knees, underarms, buttocks and back. 
Before you spend $$ on peels or IPL, try these topicals.
This is called post-inflammatory hyperpigmentation (PIH). 
I recommend Melarase AM and Melarase PM creams from Kare Skin as well as a broad spectrum sunblock like Spectrase. Bothe are from Kare Skin. Scler-x post inflammatory hyperigmentation relief complex is a great supplement to take as well. See link below. 
Spectrase is a wide spectrum sunblock SPF 50+that is also recommended to prevent ongoing inflammation.