On iOS, the grey scale filter can also be activated via triple clicking the power button:

It is the “zoom”, in which you can have 1.0 scale and add a color filter.

I like the combined grayscale and color inversion filter.

The triple click zoom is activated in Settings > Accessibility > Accessibility Shortcut: https://support.apple.com/111771

The scale can be set by activating the zoom controller once, zooming out, and then hiding it.