The colorspace is the full range that your display can, well, display. Adjusting the saturation of your display only moves things around within that range; you'll never get outside of it. Many colorspaces are also capable of displaying colors outside of our range of vision, too, but that doesn't have any bearing on this. American and Japanese TVs both ran NTSC, but Japan's version was slightly different, and now they've switched over to something else entirely (ISDB). Jacking up the brightness got you closer to NTSC-J, but there were other differences in equipment outside of the encoding that gave them a broader range of greens (which made a difference, since green is the color our eyes can distinguish the most shades of).