White Screen Test is a website that lets you change your screen to a bright white one. With a white screen, it's easy to see if your display has any dead pixels. You can also use the light from the white screen to read or do other things in the dark.
If you want to see other colors, you can do that too. You can choose from full black screen, red screen, green screen, blue screen, etc, or even select any custom color as the full-screen background.