I remember someone was arguing that it was all intentional, in that it wasn't so much due to budget but more 'artistry'.
I don't buy it.
Either way, the end result is the greatest anime ending that takes a dump on any other anime from a great height, so all's well that ends well
If it means anything, everything was actually colored in shades of grey by hand and filmed normally rather than what they usually do, which is color it and then shoot it in black and white.
Also there's two versions of episode 6,
one that returns to color at the end and the alternative version that stays monochrome all the way through.