A great artist can go both ways.
Well, yes, but there's some things that can't be helped... A whole lot of colors is fine for a tile if it's done in, say, photoshop, or actually an edited picture, but from my experience and what I've learned along the way from various other examples, too many colors in a hand-pixelled tile of small proportions = too cluttered and busy = distracting to the player = defeating the purpose of a grass tile (a typical grass tile that is), which is to actually look like grass while not drawing the player's attention away from the more important graphical aspects.
Well that's my view. :wink:
EDIT: You may want to have several grass tiles with variations; maybe use the one you have up there in the original image as the standard one and use the version with a bit of brown as a variation to break up the monotony. Usually I like to have at least three different grass/base ground/whatever tiles in any given set (usually more depending on the eviroment I'm doing) but that's in no way a requirment.