Why is my penis darker than the rest of my body?

It is normal for someone's genitals to be darker than the rest of their body. This is  due to sex hormones and extra testosterone that stimulate the production of melanin, the brown pigment in the skin. The result is for the skin on a man's penis and nipples and a woman's areolas and labia, to darken.

If there is further darkening in adulthood, then it could be a sign of an underlying chronic illness, such as diabetes.