You can also set it so that triple tapping on the sleep button brings up the accessibility menu so you can turn black and white mode on/off quickly if needed

Settings -> Accessibility -> Display & Text Size -> Color Filters. Turn Color Filters on and select the Greyscale filter.

It’s in the accessibility settings. Accessibility > viewing and text size > color filters

I have my phone on Dutch so I loosely translated those. Hope it is helpful.