Is it because it grows? Stretches? I've had some conversations about this and haven't heard a solid consensus.
If this is an incredibly dumb inquiry I apologize.
The genital region (in both males and females) contains the most melanocytes per square inch than any other part of the body. Melanocytes are the pigment producing cells in the body. More pigment = darker color.
Edit: Somebody brought up the fact that the genitals are highly vascularized, which may be the reason why the genitals are darker in color. Or it may be a combination of the two.
Heres a source for my initial statement (thanks to NotKumar): http://www.dermtext.com/common/showi.../Chapter65.pdf
Not that I doubt it but I wish the source link wasn't broken.
Another goddamn penis thread..
The scientific reason is actually that men's palms are the same color as the rest of their skin before puberty. Afterwards, though extensive and rigorous ambidextrous masturbation the palm skin actually burns onto the shaft permanently creating the darker color. Medical Science 101.