All displayed color on screen are only black and white. Why is this?

You can activate Wind down mode to reduce eye strain before sleeping and prevent your sleep from being disturbed.

ㆍGreyscale : Displays colors on the screen as grey tones.

ㆍDo not disturb: Set the device to mute incoming calls and notification sounds, except for allowed exceptions.

CF.) Night mode (Settings > Display > Night mode) also can reduce eye strain by applying the dark theme when using the device at night or in a dark place.  However the dark theme may not be applied in some apps and Home/lock screen.