It could be that greyscale is turned on in Settings > Accessibility > Visibility enhancements > Colour adjustment, or bedtime mode is enabled which has one the actions to turn on greyscale during set times.
Try either of these: 1. Search 'grey scale' within settings and turn off the colour adjustment if it's enabled. 2. If above hasnt worked, have you got dark mode enabled? You can turn it off / on via quick panel at top