This OLED burn-in test fills your screen with solid colours so you can spot burn-in (a permanent ghost of a static image) or temporary image retention on OLED and AMOLED displays. Faint outlines of a navigation bar, logo or keyboard usually show up most clearly on solid grey, red, green or blue.
Leaving the colours cycling for several minutes also works as a gentle burn-in fixer: rapidly exercising every sub-pixel can fade mild image retention. It works on phones, tablets, OLED TVs and monitors.