My 4th 900 had the same problem. Is the lower 1/2 of the screen a darker shade of color as well?
I called Panasonic and their phone diagnosis was that it was a hardware problem. My guess is that it is some sort of panel voltage issue. My unit also had an unusually loud iris that was to be replaced. I sent it to Heartland Service, they reloaded software and nothing more, and sent it back without correcting either problem. I sent it back to Heartland on the 9th of June. Within about a week of sending it in the second time, Heartland informed me that Panasonic had decided to replace the unit. As of this writing, I have yet to see the replacement, so including the first time the projector was in for service, I've been without a projector for over 2 months. Panasonic claims that they lost the projector after it was sent to them by Heartland (even though they have a record of signing for it), and that is what is causing the delay in sending me a replacement. I've recommended to their rep that their internal SNAFU is their problem, and not mine, so I shouldn't be made to go without a projector until they get their act together. They don't seem too concerned.
Let's just hope that your luck with Panasonic service is better than mine has been!