Screen displays wrong colours
Everything is greyscale, black and white
Go to Settings > Digital Wellbeing & parental controls > Bedtime mode > Customise > Screen options at bedtime and turn Greyscale off 
Everything is negative, reversed colours
Go to Settings > Accessibility > Colour and motion > Colour inversion and turn it off
Tones, hues are different or missing
- If the screen shows warmer tones than usual, check quick settings. If
- If certain colours appear diminished or their hues are different, go to Settings > Accessibility > Colour and motion > Colour correction and turn Use colour correction off
Background turned dark/light
Go to Settings > Display and turn Dark theme off