Well I mean, yeah but that's obvious. If you have a darker skin tone by design you will have darker skin on your penis. Case in point above, Mr Alencar is Hispanic and therefore his member naturally has a darker tone. As with many of the guys who's pictures have been posted - they're generally olive skinned, dark haired, Mediterranean looking etc.
I'm white, pale, blond Northern European by several generations. If I get a deep tan it makes my skin tone match, but for the most part it looks a lot darker than the rest of my body. I'm not sure there's any real explanation for it but if someone has scientific reasoning I'd love to hear it