Since IIRC Shortcuts is actually pretty clunky, it's probably easier to use Control Center to toggle grayscale, which is possible after you add the control named "accessibility shortcuts" using {settings > control center}. That is what I use.

(And on Gnome, I use and recommend the following: https://extensions.gnome.org/extension/4012/gnome-bedtime/)

On iOS, you can also set up the “accessibility shortcut” (settings -> accessibility -> accessibility shortcut), which lets you triple-click the home button to enable it.

Thank you so much for this! Not having to dig through the settings to toggle B&W makes it much more workable.

You can also have the back tap (double or triple) run the accessibility shortcut.

I use triple-back for "grayscale" (because I found myself double-tapping the back accidentally all the time.)

You can also create automations to turn grayscale back off for apps that need it. I have ones for maps, photos and whatsapp (since people send a lot of pics in my group). They run when the app is activated/deactivated.