My note 9 screen Color change most of the time when I lock it and turn it on after it or when I try to listen voice on what's app, screen Color changes and after I turn of the screen on colour become normal .
Maybe your screen is having an issue. But u can always try this method. Backup ur data and perform a factory data reset. If the issue still persists, then it's probably related to screen.
You are right, this may not be a hardware issue. But if the screen is sometimes okay and sometimes not, you can never be sure that the screen is flawless. Maybe the fault in thescreen is causing it to change colours. You should take it to the repair center but first, try factory reset and software update if available, to see if the issue is solved or not.
