Unfortunately I couldn't get that working in time (and still don't know how to do it yet).
The plan was to have the background gray and dreary, but when people get saved/cheered-up the area around them looks more beautiful/colorful. (dead trees/flowers become alive, etc). The illusion is that the sad people think the world is bleak and horrible until their eyes are opened to a more beautiful world.
Some day I'll figure that out... :D