About the Gray Screen
A neutral gray fill sits exactly between the two extremes most people test with, and that is precisely why it finds faults the others miss. On a pure white fill the backlight is at full output and drowns out small differences in brightness; on a pure black fill the panel is at minimum output and hides them the other way. At a mid-tone such as 50% gray (#808080), every pixel is driven part-way, so any panel that cannot hold an even voltage across its surface shows it immediately as a cloudy, blotchy or striped field.
The most common thing this exposes on LCD and LED televisions is the dirty-screen effect: faint vertical or horizontal bands that look like smears you cannot wipe off. They are invisible in normal content and painfully obvious during a football match or a hockey game, because a slow pan across a uniform pitch is essentially this test at 60 frames a second. If you can see the bands here, you will see them in sport.
On OLED and AMOLED panels a gray fill is the standard check for grey uniformity and near-black banding. Because each OLED sub-pixel is individually driven, small manufacturing differences show up as a mottled or pinkish-green cast across large flat areas. Manufacturers test this at low stimulus — around 5% and 10% grey — but 50% is where a consumer eye can judge it without a dim room and a measuring instrument.
Gray is also the correct background when you are judging colour. A neutral mid-gray surround is the reference viewing condition used in print and photo work precisely because it does not push your perception of an adjacent colour warm or cool the way a white or black surround does. Photographers use a full-screen gray to set monitor brightness by eye before editing, and to check for a colour cast: on a well-calibrated display a 50% gray field should look perfectly neutral, with no tint of green, magenta or blue anywhere across it.
Finally, a gray fill is the most forgiving way to look for dust and light scratches. Dust particles scatter light and appear darker than a gray field, while scratches catch reflections and appear lighter — on white the dark specks are visible but the scratches vanish, and on black it is the reverse.