Midget,
(I hope that's the correct wat you wanted to be addressed...I am taking a risk here!)
I know that FM2 and FM3A have removable (interchangeable) focussing screens (through the mirror compartment). Perhaps the FM too has a removable one? In which case you can remove it and inspect it. If that's the colored element you can get a different one.
If that is without any coloration, there are still two things that can be causing the coloration.
1. The glass in the viewfinder and
2. The mirror (as you already mentioned).
Neither of them, AFAIK, are interchangeable in an FM. That would be tough luck.
But nonetheless, I have also read that leaving a colored glass out is cooler sunlight (like by the window inside the house) for a couple of days removes the coloration. Strange! But that's exactly what I read. The sunlight supposedly bleaches the glass.
Anyway...Good Luck!
-Sachin