What people use it for
- Cleaning: white shows dust and smudges; black shows fingerprints on glossy screens.
- Video-call lighting: open it on a second screen or tablet facing you for a soft key light (the warm swatch is kinder to skin tones).
- Checking a new or used screen: white for dead pixels, black for bright pixels and backlight bleed — see the dead pixel test.
- Tracing: a tablet at full white under thin paper works as a light box.
- Reading a card or finding something in the dark.
Why a browser page instead of an image?
A page can hide the cursor, go truly full screen with no bars, and switch colours with a tap. It also respects your display’s colour management, so white is the display’s real white.
Related tests
Questions, answered
The white looks slightly yellow or blue.
That is your display’s white point or Night Shift/Night Light. Turn those off for an accurate check.
Can I keep the screen from sleeping?
Full-screen mode keeps most devices awake while the page is active; on phones, raise the auto-lock time in settings.