Open Settings
Tap Accessibility
Tap Display and Text Size or Vision settings
Tap Color Filters or Grayscale
Turn on Grayscale
If needed, enable a shortcut for quick access
On iPhone, use Accessibility Shortcut by triple-clicking the side button
On Android, use Accessibility settings and turn on Grayscale or Color Correction
If your phone has Digital Wellbeing or Bedtime mode, enable grayscale there
If your phone has Developer options or a display mode setting, check for monochrome or grayscale
Exit Settings and confirm the screen is black and white