I posted about this a while ago with the hopes of finding a way to go black and white and didn't get any replies. I still haven't found a way to do it. Some control there would be awesome now and then. I'd want a way to keep HUDs and other things in full color. Right now the only way seems to be manually desaturating textures one at a time, so I'm not advancing that idea at all for the time being.
I really don't see it being a widely used feature or needed by many players, but I'd like it.
I tried to sign it but all I did was mess up my screen.