Open Settings
Tap Accessibility
Tap Display and Text Size or Vision
Tap Color Filters
Turn on Color Filters
Select Grayscale
On iPhone, go to Settings > Accessibility > Display & Text Size > Color Filters > Grayscale
On Android, open Settings > Accessibility > Color and motion or Visibility enhancements
Turn on Grayscale or Color correction and choose Grayscale
If needed, add Grayscale to Quick Settings for faster access
Turn off Grayscale by disabling Color Filters or Color correction