[p_l]

The funny thing is, the studio claims last episode of Gunbuster was done black&white on purpose - in fact, that black&white was more expensive the way they did it than otherwise.

Unfortunately I suspect that outside of the people directly involved we won't ever hear properly. The rumours flew around since early 1990s, and there is the part that Gunbuster was originally supposed to be a TV series (AFAIK), and as result the plot was extremely compressed into those 6 episodes that we got ultimately.

And it's not like the studio was not known otherwise for wild swings even when they didn't run out of money... (anyone remembers how Mahoromatic ended?)

[justsomehnguy]

I think it's true (at least partially), you transfer the outline to a cel and then colour it, any issues are covered by the paint.

With B&W you need to transfer the shading and you can't just (literally) fill the gaps with a colour.