usually the engine bay and trunk colors remain the same unless someone did a complete strip down as part of a restoration. I don't think I've ever seen so many models of one car that have undergone color changes as Spiders
You can check on the underside of the trunk lid for a paint code sticker. It is likely that it is no longer there. If not, open up the hood and look for the VIN # stamped into the firewall. Just below the VIN # should be a metal plate with information stamped into it. It will be rivotted or screwed vertically in the firewall. It should tell you the color code. If all of that doesn't work....the color of the firewall is almost always the original color of the car. The firewall should still retain its original paint and there is your original color. Good luck!
There are some web sites that list this information. You can try http://www.fiatspider.de/e/l_3bhtm
It's a German site but has an English language section.
Good luck!
George
77 spider