The characters in Red Mars are always yammering on about the colors red and green. It's always how red the Martian landscape can be or how green their homebrewed algae are. Does no one take the time to notice a nice periwinkle on that planet? Then again, when the characters take the time to notice something time and again, this is generally a hint to the reader that they should be paying attention to it as well. So, what is so freaking important about red and green? We're so glad you asked.
First, let's have a quick discussion on color theory. Promise it won't take but a second.
Basic color theory states that when you have two opposite colors standing side-by-side, or one in the background and the other the foreground, the contrast makes the human eye take notice. In other words, they pop.
You might have noticed this trend in movie posters. Since all human skin color produces a tint of orange, many movie posters are infused with teal backgrounds—orange's opposite—to make the actors stand out. Hollywood's even started color grading teal backgrounds in their films to the point of absurdity.
Care to take a guess what red's contrasting opposite is? Indeed. (You said green, right? Because it's green.)
So red and green appear and reappear throughout Red Mars because they are opposites. Red and its shades represent Mars, a planet considered cold and dead to all except the most diehard of geologists. Green represents life as we've come to know it on Earth—think: thriving, evolving, and, if not warm, at least warmer.
Both stand in stark contrast to the other, but oddly enough, they also support each other. On Mars, for instance:
[…] both the light and dark colors were just a shade away from the omnipresent rusty-orangish-red, which was the color of every peak, crater, canyon, dune, and even the curved slice of the dust-filled atmosphere […]. Everyone felt it. (2.4.110)
This omnipresent color makes the planet stand out when compared to what the colonists are used to, a.k.a. Earth. And Michel's homesickness would not be so great if Mars were in any way like Earth. In other words, the stark red of Mars causes his homesickness to stand out—to pop, if you will—all the more.
On the other hand, Mars wouldn't contain a beauty "so strange, so alien" for Nadia and Ann if it were like the green of Earth. It forced Nadia, for example, to stop "understanding everything in terms of her past" (3.5.44). When green algae and trees are added to the palette, the characters stop to consider them all the more because they're so alien to the Martian landscape.
New Colors: Same Yinyang
But the contrast between red and green quickly moves beyond just the color palette of the landscape. It soon enters Martian society and its politics.
Those who follow Ann's view and believe Mars should be left alone are called Reds, while those that oppose her and stand with the likes of Sax Russell are called Greens. These two environmental, and ultimately political, stances oppose each other by being—what else?—opposites:
The reds say that the Mars that is already here is nature. But it is not nature, because it is dead. It is only rock. The greens tell this, and say they will bring nature to Mars with their terraforming. But that is not nature either, that is only culture. A garden, you know. An artwork. So neither way gets nature. There isn't such a thing as nature possible on Mars. (5.3.15)
As shown in the quote, by opposing each other, the two positions ultimately cause the other side's false understandings to, again, pop. But the opposite is true as well: when standing in opposition, they strengthen the resolve of the other side with the same speeches they use to defend their own. So red and green represent opposition as much as they represent bounded-ness, then. When it comes to the Reds and Greens, they're dependent on each other for their own individual existences.
Like any good yin and yang system, red and green—whether represented in the contrast of mineral/organism or political camps—must oppose each other because they're opposites. But the very thing they oppose is also a requirement for survival. Without one, the other would no longer exist.
Blend Red and Green…
… And you get brown. But when red and green represent politics and philosophy, you get something a little more complex than just another color.
Hiroko develops a religion called areophany. The principle concept behind the religion is viriditas or "greening power." According to Michel, viriditas combines "fully red and fully green at one and the same time" (4.2.86). So we see in Hiroko an attempt to combine the opposites into something whole, lacking in division and more powerful as a result.
Unfortunately, we don't get to see much of areophany, or viriditas, in the novel since Hiroko and her ideals go into hiding early on and only show up from time to time. As a result, we only get a few glimpses of viriditas, and these glimpses don't exactly solve the mystery of what it is. If anything, the mystery only deepens.
But the novel's ending suggests Hiroko was on the right track. Neither the red nor green political movements set Mars on the right track, either socially or politically. In fact, the battle of opposite philosophies straight-up wrecks the place, and at the end of the revolution, the remaining First Hundred seek refugee with Hiroko, the person who blended the two philosophies together.
This ending suggests two things. First, that in blending red and green philosophies, Hiroko has identified the best chance Mars has for fixing its problems. And second, Robinson's going to need to write a sequel to explore this issue. And, guess what? A sequel there is.