In addition to what LPChip said, there's a few other things that I think would be worth a note. First, coloring the entire map colors that can be both recognized and unique would be hard; it's a big map. Not to mention it'll look ugly. Finally, the game has no 'areas', it's a seemless world, or at least, that's what it's meant to play like, and I happen to think the map reflects that.