Dear Ms Megha
Pigmentation at site mentioned like back and forearms result from excessive sun exposure . Also many a times in order to clear the pigmentation at these sites , people use methods like excessive rubbing with brushes, pumic stone etc. Such procedures might increase the pigmentation and result in a condition called- Macular Amylodosis.
In order to get rid of these pigmentations you need to take a treatment from a qualified dermatologist
. Apart from creams , he can avice you some body peels to get rid of the pigmentations at a faster pace. Also sun avoidence and protection is a must.