Hi ! Not all glass is the rear windscreens is the same. whilst my car was not after-market tinted, I did have the rear screen replaced as the radio failed to work as it also houses the aerial.
When it was replaced it was different in its natural tint, was not a problem for me at all, but given you guys are having an issue when it is tinted you maybe suffering from colour/tint class of mixing colours, ie red/yellow make orange, or simply that when the car gets cold it causes a change in the colour, I say this because the tint effect is uniform!!!!! I doubt there is little you can do!!!
Always said I would go back to a 6 series