A neutral gray screen to check your monitor for blotches, banding or brighter patches, and to use as a neutral backdrop.
When time is up, full screen closes on its own.
Put a white or warm screen in front of you and turn up the brightness. It lights your face softly, like a ring light.
Black shows dust, white shows smudges. Use both and nothing gets missed.
Cycle through red, green, blue, white and black. A dot that doesn't change color is a dead or stuck pixel.
A soft color at low brightness works as a night light or for reading without turning on a lamp.
To check monitor uniformity. On mid gray you can see blotches, banding and darker corners.
Try mid gray #808080, then a darker one like #404040, which shows backlight bleed better.
A little is normal, especially in the corners of LCD monitors. If it's very visible, the panel may be faulty.