At a certain point, Anakin's greyness was like standing on the North Pole. You're not south or north of anything anymore, you're in the dead middle, even if only for a moment.
I feel that moment's was in Palpatine's office, where he's trying to convince Mace to keep Palpatine alive. He went from Good his whole life, Grey that moment, and Bad from then on in.
Doesn't look like a grey transit to me. Anakin is troubled, yes, but that's not displaying a moral grey area. He goes straight from good to evil in the fight scene in ROTS, and straight from evil to good in the fight scene in ROTJ.
At no point does the film even attempt to explore morally questionable actions from Anakin. There are never ever questions; he is always performing either good or evil.