Unless they are allowing multiplayer teams to choose more than just red/blue for colors, this picture then is from a co-op game. Now, if you look in the bottom right at the warthog, you will see a third spartan's visor sitting in the driver's seat. I think that Halo 2 will now allow more than 2 co-op players!
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I think this may just be deathmatch slayer or you may be able to choose the cooler of both teams.