Indeed very tough! I was thinking of the potential areas where such material could exist (in nature as well as existing technology) but could not find any. Those in technology are digital and those in nature are not monochromes. I thought of animals that have black and white vision (most animals). However, animals do not "see" things as black and white. It is their brain that processes the images and presents them as black and white. So, no such material will exist in nature, is what I think.
Then, I found this. Such glasses, although cannot eliminate other colors, can maximize one color. For example, through these glasses, objects appear more "rose" and less other colors. Similarly, what if you use black-colored glasses with a heavier tint of black. They may not eliminate other colors (so not 100% accurate) but black would essentially be more and all objects would appear a few shades darker, getting close to black and white. You could not wear them at night or in dimly lit places.