It is true Quake had a fairly greay and brown motif. It worked pretty well for that game. So why is it that so many games are Grey and Brown now? This is especially odd since the new games can allow so many more colors, but are still just using Grey and Brown.
Joystiq has pointed out that Nintendo makes sure to use as many colors as possible. So it is an interesting thing.
I guess what is happening is that these companies want to seem dark and gritty. Because of this they slowly become the same game, even if it is Halo and Tony Hawk Pro Skater.