[roughly]

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.

[StephenCanis]

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

[zimpenfish]

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.)