For those owners that haven't yet been down this road, there was an actual paint code tag on each car. Unfortunately, it was a small green-and-white decal in the front trunk- sometimes stuck on the headlight connection tube. They don't weather well. The default method is to look up under the dash to the front firewall.
Panteras were painted all over while the bodies were empty, so if total authenticity is important to you, that cabin paint behind the dash, protected from almost everything is as close as you can get. As was said, 50 years of weathering has faded your OEM paint, so old paint codes are not applicable anymore for touch-up.