Remember the angry fans that created a petition and then darkened several screenshots to their liking? Well one gaming blog managed to sit down with the lead designer of Diablo III, Jay Wilson. At which point they showed him the altered screenshots to get his thoughts on them.
He took each one and broke down exactly why it couldn't be put into a game. Essentially he pointed out that in the end they were just pretty pictures and wouldn't actually work in a game. Partially because it would make it much harder to see the monsters you're fighting.
He did state that there will be no rainbows in Diablo III, so at least they got that much. You can see the full break down of his thoughts here and if you're curious about a particular screenshot, he addresses each one individually.