Green Screen
A plain green field that fills your whole display. Check every green subpixel; also a chroma-key backdrop for a quick video call trick.
Other solid colours
For a guided check that cycles colours and explains what each one reveals, use the dead pixel test.
Questions people ask
How do I make the green screen full-screen?
Press the button. The page uses the browser's Fullscreen API. Press Esc to leave. On iPhone, where web pages cannot go full-screen, rotate to landscape and hide the toolbars by scrolling, or add the page to your home screen.
Will a solid colour screen damage my display?
Not in normal use. On OLED, avoid leaving any static image, including a solid bright colour, at high brightness for hours. A few minutes of testing is fine.