Dark knees and elbows?

im a 17yr old male, and its hot as h.e.l.l. here in Houston and i really want to wear shorts but i have these really ugly unattractive dark knees i am naturally tan but my legs are olive colored and my knees are like 10 shades darker...is there a product i can buy at like target or any store that will help with this? same goes for my elbows