The color difference you describe is pretty much standard, because the skin on the penis and scrotum usually darkens during puberty. The skin color difference when compared to the surrounding skin areas can be pretty dramatic. Guys whose genital skin color is the same as the rest of their body is not very common actually.
Take a look at the penis photos posted on this site and you’ll see a wide variation of color differences.
I’m not sure there is a successful way to lighten that skin color, except for some sort of bleaching creme that is sold in drug stores to lighten age-related spots. Usually that sort of creme is meant for removing brown spots on the backs of hands (sometimes called liver spots), but actually the result of aging.
I’m not even sure if that bleaching creme is permanent or not. If you do try it, be sure to test it on a small area of skin first to see if it is genital-friendly. It may burn like crazy.
I actually prefer the darker skin look, but that’s just me.
-damfino