Grayscale
For iPhones:
Settings -> Accessibility -> Display and Text Size -> Color Filters -> Grayscale
Create an Accessibility button shortcut to turn it on & off:
Settings -> Accessibility -> Accessibility Shortcut -> Choose Color Filters
*At the time of writing, this shortcut is used by triple-clicking the side button on your phone.
For Google Pixel / Android Phones:
Settings -> Accessibility -> Color and motion -> Color Correction -> Grayscale -> Toggle on “Use Color Correction”
Create an Accessibility button shortcut to turn it on & off:
Settings -> Accessibility -> Color and motion -> Color Correction -> Toggle on Color Correction Shortcut
*An accessibility button will appear on the right side of the screen. Tap to toggle on and off.
For Samsung Galaxy Phones:
Settings -> Accessibility -> Visibility Enhancements -> Color Correction -> Grayscale -> Toggle on color adjustment
Add to Quick Settings to turn it on & off:
Swipe down from the top of the screen twice to show Quick Settings -> Add button -> hold and drag Color Correction Tile to Active Tiles
*Accessed by opening up Quick Settings