Somtimes filmmakers use black & white for showing the darker aspects of story.
For example in the 1998 film American History X, the parts of the movie where Edward Norton's character is a racist, are shown in Black & White, whereas the part of the movie where Edward Norton is no longer a racist and is seen repenting are shot in color. Take a look here.
So my point is that use of 'Black & White' in contemporary movies do deal with darker aspects of the story. Or let's say the use of 'Black & White' adds to the dark nature of a scene or a whole movie.
Adding to that, Wikipedia entry of the movie says
"According to Six, he intentionally made Full Sequence very different
from First Sequence, due to two reasons. First, back when he was
writing the script of First Sequence, he knew people would want more
"blood and shit" than is shown, and second, the two parts reflect the
two different characters of their antagonists: the coloured First
Sequence, with a slow-moving camera, fit the story of Dr. Heiter,
while Martin Lomax's character required a "dark and dirty" film. Six
shot Full Sequence in colour, but "was always thinking about black and
white" and realized while editing that it was "much scarier" in black
So I guess, we have an idea now why it was made in Black & White.