A lot of games with 'high end' graphics like Crysis or Age of Conan for example, have APPALLING color depth.
"High-end" graphics has nothing to do with colour depth, per se. I think that it has much more to do with:
(1) the "photo-realism" effect that those games were aiming for (as opposed to the "fantasy" setting of WoW); for better or worse, Age of Conan was aiming to be "dark and gritty", for instance; and,
(2) laziness on the part of the developers. For example, I think that both Age of Conan's and Crysis' opening levels were fairly colourful, but the colour tended to wash away as the game wore on, suggesting that the graphics artists spent more time at the beginning than the ending.
(In contrast, I was fairly impressed by a recent Star Wars: The Old Republic walkthrough video which showed a really vibrant, interesting universe.)