I think you should apologize to grey and brown for insinuating that they are not colors, you might have hurt their feelings!
On a more serious note, I think the game is beautiful so far (I've played about 20 hours, mostly following the story). It's clearly inspired by Nordic countries during early medieval times, and the atmosphere seems spot on. Most of the colors are inspired by winter and fall, with some green thrown in, but it works, and looks very good. And there are actually flowers here and there, mostly blue and white so far. So the colors are mostly brown, grey, white and green.
One or two areas with more colors would perhaps benefit the game, but on the other hand, I think it's commendable that Bethesda went so far in creating a melancholic and unique atmosphere. Actually it doesn't feel that melancholic all the time, because the world is quite lively in places.
So far I'm really enjoying the game. The biggest flaw, in my opinion, is that while the world itself is fantastic, the rest of the game is "just" very solid. Neither the story, the characters or the gameplay have brought any "big moments" so far.
