That sounds like wind down mode. Go to your settings > digital well being > wind down. See if its triggered on. If it is disable it. This turns your device into a gray scale (black and white) mode. Let me know if this isnt the case and i can think of another solution.
just thought of another setting to check incase its not wind down. settings > accesibility > visibility enhancements > color adjustment > grayscale (if its on you can just turn it off)
