Purple screen fix as it is spreading

If it's an OLED or AMOLED display you are talking about, you need to transfer your data to a different device or replace it ASAP as the display has oxidized. Oxygen has gotten inside the OLED (likely due to a microscopic crack from a fall or hit) and it will just keep spreading over the next few hours/days until the whole screen is purple so i suggest you take action immediately. I've seen it happen many times. Sometimes the screen is gone in as little as a few hours and sometimes it can take weeks. But it's better to be safe and back up or move your data from the device sooner then later.