I created a shortcut to toggle grayscale for iOS. Try it out!
https://www.icloud.com/shortcuts/371d5ced520d40e984d626ae304...
https://www.icloud.com/shortcuts/371d5ced520d40e984d626ae304...
(And on Gnome, I use and recommend the following: https://extensions.gnome.org/extension/4012/gnome-bedtime/)
I use triple-back for "grayscale" (because I found myself double-tapping the back accidentally all the time.)