Considering the true white car is in poor cosmetic condition, that will be the basis for the car's value not the original paint color. I'd think this would be more of an issue for something like a 63 Vette show car than a 96 850R. What you can do is talk to a top quality body shop to see about their abilities to custom mix paints such that the new parts match well to the old. Upper parts like roof, sedan trunk lids and hoods weather and fade more than doors, valences and air dams. Also ask your body shop for a quote to clean up the existing body and where they'd recommend going with a new clean part. If the car is worth a makeover, then this should give you a data point for the effort to do it right.

 

In my opinion, money better spent on a newer R that is clean and ready to go.