For anyone on Android, you can bind holding down both volume buttons at the same time to toggle grayscale on/off.
To do this, go in Settings, then `Accessibility > Color and motion > Color correction`, and set it to grayscale.
While still in the `Color correction` submenu, click `Color correction shortcut`, set it to `Hold both volume keys`, and toggle it on.
I just discovered this myself, but with the ability to quickly flip it off when needed (taking pictures/watching videos/etc) I may actually end up keeping it enabled this time around.
To do this, go in Settings, then `Accessibility > Color and motion > Color correction`, and set it to grayscale.
While still in the `Color correction` submenu, click `Color correction shortcut`, set it to `Hold both volume keys`, and toggle it on.
I just discovered this myself, but with the ability to quickly flip it off when needed (taking pictures/watching videos/etc) I may actually end up keeping it enabled this time around.