Hi - well, my first thought on that would be to turn the saturation on your display all the way down to 0, but that might not be possible on laptops, or some standalone displays!
I’m not aware of a built-in software method for making the screen grayscale, but if you’re willing to investigate the security of it and potentially use it, there’s an add-on that claims to do this: GitHub - lliurex/kwin-grayscale-effect: KWin Grayscale Effect