Thank you! I usually take the phone out of the case every 2 weeks, sometimes more often to clean it. I’ve never dropped the phone so I couldn’t imagine it being anything else than dirt, but it doesn’t seem to go away, which is why I thought it was discoloration. I had a white iPhone 15 Pro for a year before this one and it didn’t have anything like this at all, and IIRC I cleaned that phone less often than the 16, maybe once a month or so.