Good question, but mine isn't...
?? Piet, look again at the first photo in your gallery, "waking up." compare the color of your cock to your untanned skin nearby. Your cock's a lot browner than that skin is.
It has to be two things. There is more skin there like bassoon said. Second, the skin type is different, since it's very elastic. Not true for skin elsewhere, which develops stretch marks, ie scars when it expands too much. Guys' cocks and balls are always darker than the nearby skin, even on the darkest guys. That skin has to expand and contract constantly, without any damage.
The less pigment you have in your skin generally, the more translucent it becomes. That means you can see the blood vessels more easily, making skin itself look more pink than pigment colored. 30,000 years ago, infants and adults in colder climates would have been able to absorb vitamin D more easily when they were bundled up if those small parts of skin (faces, hands) had less pigment, and were lighter rather than darker. So survival rates improved up north for those with less pigment. This is why migrating african tribes gradually lost their original skin color living over long periods in less hospitable climates. People probably thought there was something magical going on if a lighter skinned infant survived, and a darker one did not. But it was just vitamin D.