Use InjuredPixels to screen test a smartphone or tablet in the store, before buying, or during the warranty period, as you may get a replacement according to the manufacturer's dead pixel policy. InjuredPixels fills the entire screen with a primary or custom color, helping you to easily spot pixels that do not match the selected color.
The app is very easy to use: tap the color buttons or use the volume keys to cycle through colors; double tap anywhere to hide the buttons and check every inch for defects; double tap again to show the buttons; swipe down from the top of the screen and tap Back or Home to exit the app.
When going full (empty) screen, touching, tapping or swiping does nothing, giving you the freedom to clean or gently rub any area while testing.
InjuredPixels is completely free, 100% open source, doesn't show ads, and requires no Internet and no extra permissions whatsoever.