I have made some quick comparison between my Paperwhite and friend's K4 (silver one). With light turned off I couldn't see any difference in contrast - PW screen looked better thanks to increased resolution, but that's all. With lights turned on, there was difference - PW screen was really whiter.
I don't care about shades at the bottom of the screen. Why? Because I found out that when shades are visible, that means light level is too high for environment you're in. When it's really bright (outdoors in sun, for instance) light is hardly visible - sometimes it's not visible at all, sometimes setting it at max level makes screen a little bit whiter. You can't see any shades. When indoors, I always adjust light to highest possible level on which shades are not visible yet and I found this strategy to be working great, always providing correct light level which is perfectly readable and not tiresome for eyes.
So in my case shades are feature, not bug