I would say it's almost impossible to kill a mushroom, heh, even in the dark, they just cling to life and rebound quickly. As for your "bleaching" I have noticed that mushrooms, when moved under intense lighting, will develop a protective "sheen" of sunscreen, which maybe what you're looking at. The cap will also not expand as much to limit its sun exposure, and when contracted, it would look richer in color, under the "sheen." I hope you get the drift of what I'm trying to describe and I wouldn't worry about it.