The color code names seem random, but the point is that Joe insists they not know each other's real names. That way, if they're caught, they can't rat on each other because they don't know anyone's identity.
The suits serve the same purpose. All the gangsters wear identical black suits, narrow ties, and sunglasses, with their hair slicked back. Both these plot devices represent the anonymity that Joe thinks is necessary for a successful job. They're wearing the suits in the diner scene where we can see from the dialogue that these guys aren't all the same by any stretch. Once the heist starts, though, the idea is that everything's strictly business. No need to get personal.
Tarantino said that the suits do double duty. Not only do they make the guys look cool and film noir-ish, but he claimed they add a note of realism—some robbery jobs have been done by guys dressed in identical outfits to avoid being ID'd.
We don't know how true that is, but it's a popular plot point in films. Think about the nun costumes in The Town, the clowns in The Dark Knight and The Killing, the white jumpsuits and bowler hats (so creepy) in A Clockwork Orange, or the painters' uniforms in Inside Man. Tarantino chose black suits and ties, an homage to the film noir genre that he loved and borrowed from in many of his films.
Oh, one other thing: that building they're all holed up in? That's an abandoned funeral home, complete with folding chairs for the memorial service and a hearse under a tarp. So yeah...those black suits? These guys are all dressed up for their own funeral.