Even if you've got a good handle on the admittedly complicated backstory to Blackest Night, that still leaves seven new Corps to keep track of. The colors themselves are actually fairly easy to remember, as they're the same as the colors of the rainbow. That's right - it's time to bust out the old ROY G BIV mnemonic, because it's pretty much the entire key to Blackest Night. Well, that and the black light of the Black Lantern Corps. That's probably important as well.
In an interview with Publisher's Weekly about a month ago, Geoff Johns provided one of his clearest explanations for the mechanics of the various colors. Each of the seven represents a different aspect of the emotional spectrum, something that sentient life created simply by existing.
Green is the central color, a fulcrum point between the three negative emotions (the ROY half) on one side and the positive emotions (the BIV half) on the other. Its status as a pivot point is partially why, despite the colors representing an emotional spectrum, the stated quality of the Green Lantern Corps - willpower - isn't technically an emotion at all. Instead, it's the color of life itself, which balances out all the other colors. So, let's take a look at the various colors, and we might as well go in rainbow order.
Red: The color of rage, red is easily the most chaotic part of the emotional spectrum. The Red Lantern Corps was brought into being by Atrocitus, who in the recent "Secret Origins" retcon arc in Green Lantern was revealed as the killer of Abin Sur and the archenemy of then Green Lantern Sinestro. Billions of years old, Atrocitus was one of the five survivors of Sector 666, which was massacred by the Guardians' robotic servants the Manhunters due to a programming error. (It was this tragedy which led to their replacement by the Green Lantern Corps.) Upon induction to the Corps, Red Lanterns have their blood replaced by the rage energy of the Red Power Battery. This generally makes them creatures of pure instinct, and it has the added bonus of allowing Red Lanterns to shoot liquid rage out of their mouths as a projectile weapon.
Orange: The color of avarice, the orange light has only one wielder, the ageless thief Larfleeze. Though Hal Jordan compared his looks to the Muppet Gonzo (I guess I can see the resemblance), Larfleeze and the Orange Power Battery are really just the Gollum and One Ring of this War of Light. He jealously guards his precious treasure, living in complete isolation on the assumption that anyone he encounters is out to steal the orange light from him. The Guardians of the Universe negotiated a non-aggression pact with him eons ago, stating he would never unleash the orange light on the universe in exchange for the Guardians leaving him alone, which led to the longstanding prohibition on Green Lanterns entering the Vega System, where he resided. Recent events have seen this pact shatter, and Larfleeze (or, as he also calls himself, Agent Orange) has unleashed the terrible power of the orange light, which allows him to create deadly avatars of those he has slaughtered, allowing him to create entire Orange Lantern armies from his victims.
Yellow: The color of fear, yellow has been a major part of the Green Lantern mythos since a dying Abin Sur warned Hal Jordan of the yellow impurity in the very first Silver Age Green Lantern story. The role of yellow has evolved quite a bit since then, and it's a fairly long story (I tried my best to recap it all over here). Heading into Blackest Night, the Sinestro Corps is undergoing a leadership crisis, as the alien warlord Mongul has usurped Sinestro's rightful place as leader of the Corps (I mean, the thing is called the "Sinestro" Corps, after all). Having escaped his death sentence for the umpteenth time, Sinestro has a pretty long to-do list at the moment, which include reconnecting with his long lost daughter, challenging Hal Jordan to confront who he really is, further his absurdly complicated masterplan (whatever it is), and regaining control of the Sinestro Corps.
Green: The color of willpower, the role of green in all this is hopefully fairly obvious at this point. One thing that might be worth keeping in mind is the very fact that willpower isn't an emotion, as some critics have pointed out. The Guardians of the Universe very explicitly rejected emotions billions of years ago, and their recent actions have only driven them further down the path of senseless devotion to supposedly logical principles. Blackest Night will certainly deal with the legacy of the Guardians' rejection of emotion, and I suppose it's possible green may ultimately take on a more clearly emotional character as the new status quo slowly comes into focus.
Blue: The color of hope, the blue light is controlled by the now renegade Guardians Ganthet and Sayd, who abandoned their comrades to pursue a path in tune with their natural emotions (which included, among other things, their love for each other). The Blue Lanterns have no offensive capabilities of their own, but can instead allow Green Lanterns to experience vastly heightened power levels and instill in their comrades the hope needed to attain victory. The Blue Lantern Corps grows slowly, as Ganthet and Sayd devoted their time entirely to finding one being from Space Sector 1 worthy of the blue light, who in turn would then spend his time finding a suitable candidate from Space Sector 2, and so on. (Considering there are 3600 sectors, this could take a while.)
Indigo: The color of compassion, that's almost all that is known about those who wield (or, as the Ganthet puts it in Green Lantern #25, "weave") the indigo light. Like the color indigo itself, this corps has been the forgotten faction in Blackest Night's rainbow of warriors. All we really know is that they're not technically a corps so much as a tribe, and that the Indigo Tribe is led by a woman named, well, Indigo. I don't mean to sound glib here, but that's a whole lot of "indigo" right there. Anyway, they're also unknown to the DC universe at large, the image of them in Blackest Night #0 suggested tattoos and body paint are an important part of their appearance, and it's thought that the Blue Lanterns will need to form an alliance to them. The miniseries Blackest Night: Tales of the Corps, which unfolds over the next three weeks, has promised to reveal more.
Violet: The color of love, violet is controlled the Zamarons, the female counterparts to the (mostly) male Guardians. In past incarnations, the Zamarons had granted power to women through violet crystals that contained their psionic energies, turning whoever held the crystals into the superpowered Star Sapphire. The most notable Star Sapphire was Carol Ferris, Hal Jordan's one-time girlfriend and probable true love, who was possessed by the crystal on numerous occasions and forced to fight the Green Lantern. Geoff Johns has reconfigured the Zamarons and their violet power to better suit the notion of a corps along the lines of their Green Lantern and Sinestro counterparts, complete with its own energy entity along the lines of Ion and Parallax, which has been named the Predator. Thus the Star Sapphire Corps has now appeared, which thus far has been exclusively female. Carol Ferris has accepted a position in this new corps, as have several other women from throughout the galaxy who have lost their true love. The Zamarons are also recruiting from less reputable places, as they have begun using the emotional energy of the violet light to reform several female supervillains, including John Stewart's former nemesis Fatality, lone survivor of Xanshi, a planet Stewart allowed to be destroyed in a regrettable moment of arrogance.
Black: The color of death (assuming you consider black a color at all), the black light flows forth from the Black Lantern Power Battery, formed from the husk of the Anti-Monitor in the aftermath of Sinestro Corps War. The Guardian known as Scar, so-named for the injury she suffered at the hands of the Anti-Monitor during the conflict, has become enraptured by the black light, and is now a servant of Death. The Green Lantern villain Black Hand, previously best known for his device that could mimic the abilities of power rings, was retconned in the "Secret Origins" arc as a witness to the first battle between Atrocitus, Sinestro, and Hal Jordan. Atrocitus recognized in Black Hand, then simply mortuary assistant William Hand, a gateway to absolute blackness that existed within him. In Green Lantern #43, he finally made full contact with Death, killing his family and at last himself as the final part of his journey towards death. He arose as the first of the Black Lanterns, designated by Scar as the herald of the Blackest Night.