Panasonic PT-45LC12

I have a 4 year old Panasonic DLP and there is a rectangular shaped discoloration (yellow) in the middle of the screen.   It is a failrly large area (about 24 wide by 12 high).  I have replaced the bulb and it did not fix the problem. It never moves or grows and has become a distraction now that I know it is there. Any thoughts?