Actually if they can interbreed and produce fertile offspring then, yes, I'd say that they are human. That's the (most common) definition of species. The separation of some inter-breedable species here on Earth (e.g. wolves and coyotes) are historical legacies that are officially listed as exceptions by the ICZN, etc. You'd have to specify that all half-orcs are mules to claim they are similar but separate species. I guess then that it makes sense that we usually refer to these as races rather than species.
Granted, there is weirdness like hybridization and ring species. Actually, making all of these fantasy races a ring might be interesting...
My understanding was that primitive hominins co-existed for so long because for most of their existence they were essentially pre-technological, or at least minimally-technological, such that they existed in niches, more like animals than like modern humans. Heck, it took genus Homo a million years just to leave Africa. But once behaviorally modern humans developed (about 50,000 years ago IIRC) they eventually wiped out or assimilated all the others. Not necessarily directly, mind you, but probably by out-competing them. An intelligent and highly social-cooperative sophont is just too damned good of a generalist.
So, one interesting idea would be to have the orcs, elves, humans, etc., all be either created or "uplifted" from niche primitivism relatively recently (i.e. somewhat <50k years ago) if you want a realistic explanation. In such a case, right now they could still be in the process of competing with one another, but there has not yet been enough time passed for a clear winner to emerge. This would be a good explanation for the fantasy trope of elves and dwarves as "dying races," and also that they still favor their niche somewhat. In fantasy such "uplift" is easily explained by the intervention of gods, as of course is creation. "Lo, before you is the likeness of Quanlominil, the Elven Lord of Fire and Bronze! And there is the statue of Llorafendim, she who taught our people to preserve words as art rather than lose them upon speaking!"
Perhaps the pantheons of the different races have a Great Wager going regarding whose chosen people will prove victorious. I could imagine a large host of primeval gods breaking out into "teams" (i.e. pantheons) who then shape the first "clay men" into humans, elves, orcs, dwarves, etc. for the Wager. And perhaps some who aren't doing well are sore losers...
Regarding skin color, the wikipedia page on human variability has a map that provides some small insight. But one has to wonder why elephants aren't black if the human model is universal? So, clearly skin colors other than the human melanism spectrum are possible.