I do agree that other Chapters would be lower-hanging fruit; Blood Angels (apart from the seemingly particularly powerful transformative effect of Sanguinius' geneseed), Imperial Fists and Dark Angels (the latter two recruiting from across the Galaxy) would all be more likely to have a dark skin tone.
There was a black Catachan model in the 4th Edition Imperial Guard codex (as well as a whole Regiment in the Doctrines section in the back of the book who hailed from a volcanic world, the Kanak skull takers).
Ok, I want piebald/skewbald Marines. That is an awesome idea. It does happen in humans naturally in real life too, but it is super rare (although the underlying process is surprisingly common).
I think the violet irises of Cadians is a great example of this- just extra diversity that we don't have currently. Presumably filters out Warp radiation bleeding form the Eye. I believe Ratlings are an example of low-grav abhumans.
I would've thought that the initial colour of the Marine prior to implantation would still affect their tone afterwards though, it would just fluctuate to a greater amount with the melanin-producing capabilities of Marines as compared to normal humans. I doubt the vitamin-D production is really an issue for Marines, they likely get all the nutrients they need through their diet.