The first thing that sprung to my mind was the amount of time it takes for them to adapt; two centuries is nothing. This will be okay though, as long as you say something about Mars' light, dust, etc. made them that way. Otherwise, it's way too quick. It would also be quite interesting to find out what has happened to black people now that a new race has formed, because there is obviously still racism going on today. Would they be shunned even more, and considered as strange as the new race? Would white people "realise" that black people are humans too and give them equal rights (of course we're meant to have equal rights today, but just take a look at the world and you can tell that isn't true). Also, how do the Martians react (I also think it would be cool, if that's the right word, to have the word Martians without a capital letter - the "real" humans say they're not worthy enough to even have a capitalised M)? Do they make war, hide themselves away, or what? I think by making a new race you can show just how racist the human race is. It sound like a very interesting idea.
Will there be any cultural exchange? Will Martians be (physically) able to live on Earth, or they become too adjusted to lower gravity, etc.? If the Terrans are like a majority, and Martians are a specific kind that only lives on Mars, I don't think Terrans would think of them as an abomination. More likely they would just have a lower opinion of them, thinking Martians are provincial, etc., but wouldn't see them as danger. On the other hand, the Martians themselves, as a minority, would probably feel threatened by Terrans' plans for dominating the universe. So, I can imagine some Martian radicals starting the violence, and then Terrans might feel threatened and decide to do something about them. Or maybe the Martians didn't develop gradually (I agree with Thomas Kitchen that two centuries are too short for any significant changes, maybe apart from the impact of lower gravity), but helped themselves using technology. Things like DNA manipulation and whatever scary body improvements you can come up with might influence public opinion quite badly.
Yeah, I think there would also be a lot of coming and going as well. I can't see a Mars cut off from Earth, as long as they're both in existence. If there was travel between the two, or even if the travel was mostly one-way (Earth to Mars) there would be enough genetic new blood to keep any distinctive 'race' from developing. The one 'characteristic' that someone born and raised on Mars would be likely to develop would be adaptive size. Lower gravity would mean probably a weaker heart, taller, weedier stature, weaker muscles, etc. I might be havering here ...so let scientific people jump in with the corrected version of what I've just said! ER Burroughs lived and wrote during a time period where scientific knowledge of Mars was pretty much nonexistent, other than it showing red on telescopes, with a bit of landform visible in the more powerful ones. It's a mistake to try to apply his fictional ideas to 'real' Mars. Back then, what he was writing was really fantasy, not sci-fi.
So, other than the timeframe which has already been hashed out here for the formation of a new race*, you would need to explain why the red skin. Humans don't take on the color of the dirt in the areas they inhabit, so that red skin would need a reason for being. * Current thinking is that it takes between 25 and 35 thousand years for a true race to develop because of environmental pressures. This is a quite different phenomenon than say, the result of mixed races eventually coming to a median phenotype.
I was thinking about the pigmentation thing too. The surface is reddish/butterscotch-y for reason X. What has affected human skin color development on Earth is the need to regulate the amount of ultraviolet penetration. Is that why the surface of Mars is red? (I haven't the foggiest, but I kinda doubt it). On the other hand, this doesn't have to be scientifically valid, it's fiction after all. Racism: well, if Earthicans go to Mars, would they be discriminated? I could see the future café latte race of Earth consider their skin tone lovelier than that blasted crab red and perhaps they'd indeed, discriminate against Martians. However, racism isn't always based on color, is it? In my country dark Somalis, white Russians, impossibly white Swedes, and in-between Romanis are equally discriminated. There are other factors too you'll have to take into account when addressing the question of racism in your story.
The surface of Mars is red for the same reason large swaths of Australia is, iron oxide, a.k.a rust. And though you are correct that you can gloss in fiction, for me there is a threshold. By that I mean, if you go so far as to explain some parts of dynamic X in detail, if you come short or get it plain wrong, that flaw shows rather much. If you don't try to explain it (warp drive) we accept it as a gloss and don't ask more.
Ah, now you have something here that I like and can sink my literary teeth into because it also give a motivation for the exploration of racism. If there is, as you have mentioned, an element that artificially accelerating mutation rate, then the Martians could easily select for color with a purpose that is consciously driven, not environmentally driven. Even this kind of selection would take some time. You might get a fairly homogenous red race of people close to the end of the 700 years you are planning.
Well, a mutagenic compound is not going to be so selective. You could write it that way, but then you take away some of the power of the Martians wanting to be different, wanting to be a recognizable people. Let them select for it, then you can have other layers of complexity to exploring racism. You could have those who don't want to select for it, who feel it's not right, not beneficial. This gives you the opening to the dynamic of racism within a race, which very much exists.
Find "The Martian Death March", by Ernest Kinoy. You might not be able to find a print version - it was aired as an episode of the radio science fiction series X Minus One. When you listen to it, you will see parallels to how the American settlers from Europe treated the native Americans.
Genetics: It takes thousands of years for big changes like racial features (BTW, human differences are insignificant in terms of genetics, we are all one race, our differences are ethnic). I like the idea of manipulated genetic changes to speed things up. But body size has changed in much shorter time frames. Only a century ago people on the American continents were a few inches shorter on average. The mechanisms behind the changes are probably more epigenetic than purely inherited genetics. A gene can be turned on or off more easily than a new gene can evolve and spread into a population. I believe you could expect to see population changes due to the lower gravity on Mars within only a couple generations. Dark skin evolved more than once, growing lighter as humans migrated toward the poles and darkening again as they migrated toward the equator on new continents. On Mars people would be light skinned given the low level of Sun's rays, however, it's possible some other mechanism for producing vitamin D might evolve, or humans might, at least for a time, be dependent upon any number of artificial replacements for micronutrients that we acquire naturally on Earth. In fiction one has any number of scientific realities to play with to get the 'race' you wanted to get.
Remember that evolution happens because some people are able to survive and reproduce, and some are not. It's about death and reproduction. So in a fairly technical environment where people are sheltered and have medical care, you're not going to see accelerated evolution, and in fact you're probably not going to see environmentally-influenced evolution at all. Unless you find a set of plot elements that mess with life and reproduction. So: Mars has some environemental condition that kills people unless they regularly take some drug or dietary supplement. The supplement turns their skin red, and the longer they take it, the redder it is. It also makes most people sterile. A small subset of people have a genetic advantage that allows them to take the supplement without being made sterile. That genetic advantage happens to be associated with black hair. Ta da!
What about deliberate genetic engineering? I could easily see that leading to the perception of a lesser race in the eyes of the "naturals".
Be that the case, that there are ethnicities and races within the Martian group itself, it will be important to know about hypodescent, when and how it applies.
How about the gravity of Mars? I remember it was different to Earth's which directly affects landing and rocketing out of the planet with a spaceship. Gravity could affect the height of people over time, couldn't it? Unless humans somehow manipulate it. In this one science mag, there was an issue about how humans would evolve if we lived a longer time in space. I think the skin was supposed to grow thicker due to radiation or something, but I can't find the article now. 'Earthicans' is from Futurama x) Well, it's probably been used elsewhere too...
Yeah, I may have missed that bit. Gravity -- or lack thereof can be such a joykill. Luckily, science doesn't seem to be able to really explain it, so there's some leeway to how to employ it in a fictional setting.
Also (and you may have covered this already, but just in case...) the Martians are going to have to live inside bio-domes or underground. Terraform the surface all you like, there is one glaring problem from which Mars suffers as regards making it habitable: no electromagnetic field. Mars is cold inside, no molten core to spin into an electromagnet. The Earth's EM field has saved us uncounted times from being fried by solar ejecta and emissions. Even as far out as Mars, with no EM field to shield them from an ejecta hit, they would not last long at all.
