XPost: comp.os.linux.misc
I found this script project to toggle a grayscale desktop on and off. I used the compositor method it supports (with the picom compositor). It doesn't affect the root window wallpaper (though there may be an option for that). There are other methods besides using the compositor that can be used, though I have not tried them.
The shell script file is instructive.
https://github.com/bubbleguuum/toggle-monitor-grayscale
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