I have a used G37S and there are rectangular film patches behind the doors by the wheel well.

What are these discolorations and what to do about them?

 

Fillet Protectors

Best thing you can do is remove them and either replace (costly) or find a car vinyl shop that can maybe fabricate a replacement. Nothing can be done about the "discoloration." They are supposed to protect the area from "rock chips."

 

Use a hair dryer to loosen the plastic and adhesive, they will come right off.

Even if they aren't discolored, it looks better without them. However, you will get pain chips where those patches go. Guaranteed.

 

Mine did not come off easily, in fact, they are still partially there.
i've added the oem sport "mud guards" and haven't had a rock chip issue yet

 

helicopter rotor/aircraft leading edge tape will last nigh on forever as a replacement. Pricey though.