Well, obviously you can just open the screenshot with Photoshop, GIMP or whatever you use and make it black&white afterwards.
However, if you want it in-game, you could use a PostMagic LUT. There is a "Desaturate" LUT that is included, that might be what you are looking for. Also various other people made LUT packs you could check out. "LUT" stands for "LookupTable". Its just an image that contains pixels for every color, so PostMagic can check, for every pixel on screen, what particular color its supposed to be. Basically take the Neutral LUT image into your favorite image software, apply some color transformation like desaturating, contrast, brightness, etc. and save the image again under a new name. Then load your LUT and PostMagic will apply the exact same operation in-game. Its a simply color remapping, note that you can't do anything that depends on neighboring pixels, like blur.
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Admin Note: This plugin is specifically made for desktop use and as stated in the description only partially works in VR. If you wish to not have this plugin loaded at all you can permanently disable it from within the "Package Manager"...
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