What a full screen of color is good for
Filling a display with a single flat color sounds trivial, and that is exactly why it is useful. It removes every variable. Whatever you can still see is either dirt on the glass, a fault in the panel underneath, or light coming from somewhere else in the room.
Cleaning. A white field turns fingerprints into dull grey clouds and makes dust obvious. Normal desktop wallpaper hides all of it. Finding faults. A dead pixel stays dark against white, while a stuck pixel is jammed on and shows against black. Single-subpixel failures only appear on the matching red, green or blue field, which is why a proper display check cycles through all five.
Light. This has quietly become the most common reason people reach for a tool like this. A phone, tablet or laptop filled with white is a genuinely good soft light source, because the panel is large relative to your face. That is what makes the light soft and the shadows gentle, and it is what photographers pay real money for.Mood. At low brightness the same screen becomes ambient light: a warm glow behind a desk, a cool wash behind a television, a color that changes the feel of a room.
Getting a clean result
Two things get in the way. Interface is the first: any toolbar, cursor or notification breaks the field, which is what fullscreen mode is for. Your display's own color management is the second. Night-shift modes and blue-light filters tint everything toward orange, and adaptive brightness can change the level under you mid-task. Turn both off before you judge a panel.
Nothing you pick here is sent anywhere. The tool runs entirely in your browser, and the color you choose lives in the address bar, so sharing the exact screen is just sharing the link.