If you want to match an existing paint, the only way is to get it scanned, paints fade over time.
Anders Hellberg in Sweden who keeps the Gr3 registry had found the color through a guy that painted at the factory, but I don't know he ever posted the actual paint code. It is a metallic black with a matte clear coat.
His username is #7395 GR3 -75 if you wanted to contact him.
Julian
PANTERA INTERNATIONAL
A DE TOMASO CAR CLUB
Presents the De Tomaso Forums
The On-Line Meeting Place for De Tomaso Owners and Enthusiasts From Around the World
Clicking on the banner will take you to the sponsor's website.