[kcartlidge]

On my iPhone SE 3:

- Accessibility -> Display & Text Size -> Colour Filters -> Greyscale switches it on. It also sets the default colour filter which is helpful because ...

- Accessibility -> Accessibility Shortcut -> Colour Filters means you can then triple-press the Home button (on an SE) to toggle that setting at any time

As various studies have suggested in the past, I do find I use it less in greyscale.