Why did my phone display change to grayscale?
All devices running Android™ 9 and higher have a bedtime mode feature.
When this feature is enabled your phone will change to black and white, as shown in the figure below.
See Also
How to change the color of widgets?Change your display theme - AndroidHow to Add Widgets to Your Home Screen on Android: Easy GuideHow to change app icons on Android
To turn off grayscale:
- Go to Settings > Digital Wellbeing & parental controls
- Touch Bedtime mode
- Touch Customize
- Turn off Grayscale
To temporarily switch back to regular colors from grayscale, open quick settings and touch 
Check to ensure color correction is not enabled:
- Go to Settings > Accessibility
- Touch Color and motion
- Ensure Color correction is Off
