Actually the Gurren producer/Gainax co-founder, Takami Akai, was the one to quit, due to his snarky comments against 2ch. Osamu Kobayashi (4th episode director) was likely just meant to be a one-shot episode director in the first place.
Anyway, not much to add about Gunbuster other than it's not automatically easier to do something in black and white. Some might argue it's harder, because all you're working with is shades of gray.