Aftermarket paint job on a 355?

How much does a very high quality aftermarket paint job knock of the value of a 1999 355 spider? The cars exterior was changed from original factory yellow to factory silver. This was a very high end $12,000 paint job. It was done because the previous owner did not like yollow, not because of an accident (this was verified). Thanks for your thoughts. The interior is still factory black. Tim