I've had my Paperwhite since they were first shipped. It has never had a "bad" screen.
Recently it started to show some small signs of color. It has always shown the little cones of light from the LED bulbs but this was new.
I dug out my eyeglass cloth and cleaner, shut off the PW and carefully wiped the screen with the cloth after spraying some cleaner on the cloth, not the screen.
Voila! My PW is back to essentially perfect. Others should try this and report. I have a funny feeling that this is the answer.