Our skin colour is regulated by several factors - most importantly and predominantly one's genes. The area of the genitals - has high number of certain receptors that cause it to become more pigmented than other areas. This just means that their receptors and hormones are functioning normally and they do not have any disorder related to pigmentation. Even along the evolutionary process - most commonly - areas of sexual importance are more pigmented and differently pigmented in most animals and humans. So dont be afraid or bothered. There are no medications to permanently change the colour of your genitalia. IF THIS STILL BOTHERS YOU - PLEASE SEE A QUALIFIED DERMATOLOGIST NEAR YOU. Warm regards, Dr Govind Suresh Mittal.