Full screen uniformity check: one flat color, edge to edge

Open the tool and tap the Fullscreen button in the top right corner of the color area. The controls then hide behind a small toggle in the bottom left corner, so tap it to open them. Use the hex field in the panel to set the color. Start with the page color, a dark gray (#333333), then try a mid gray (#808080), a light gray (#CCCCCC) and white. Dim the room for the dark fields, and keep the room light the same for each shade. Note which shades show a problem and where on the screen it sits.

Look for four things: a patch that stays in one place, glow along an edge or corner, a color that shifts from one side to the other, and a blotchy or clouded texture. Look from your normal distance first, then step to the side and move a little up or down. Patches that change as you move are a different problem from patches that stay put. For what the patterns can mean, read the gray uniformity and dirty screen effect guide, the backlight bleed vs IPS glow guide and the OLED near-black guide. Full screen is local to each device, so each screen in a session needs its own tap.

This check is visual only. It can't map uniformity with numbers, and a flat full screen color isn't the same as a test with real content, like a dark movie scene or a game. If a patch shows up in everyday use, check the maker's policy on uniformity and the seller's return window. A colorimeter or other measuring device is the way to get numbers, and this tool doesn't do that.