Well, there is some history behind the whole thing...
Originally, the blue and orange was the logo of a US
oil company that Gulf bought. Wyer and Gulf wanted to use them on the race cars, so they used the English equivalent based on a British Leyland (Standard-Triumph) code called Marigold. They used an ICI code 3393. and that was the original Gulf orange from '64 or so. The primary paint companies in the UK at the time were ICI, Glasurit, and Berger, and there were a few more in Europe like MaxMeyer.
Anyway, the original is ICI, the PPG version is a cross reference to the ICI...PPG bought ICI in the late 90s.
By the 70s PPG was a player in the UK, and the Topaz is the same as what PPG thinks is Gulf Orange. The Porsche Gulf Orange is a different code. I have seen both and cannot say there is a difference, but I have never seen them side by side.
So the only way to get the original orange is to get the ICI paint which no longer exists. I think the PPG is a great match. Funny thing is, I had painted out some samples and took them to the Festival of Speed in England years ago, and Jacky Ickx (of all people!!) saw me kneeling in front of a Porsche 917 comparing my chips to the real thing. He laughed and said they were about as perfect as you could get.
Take it for what it is worth, but I would have no problem using the code I supplied and would be confident that it is as close as possible without finding a NOS gallon of the stuff somewhere.
Hope that helps, and I hope you dont mind the history...its a bit of a hobby...
Cheers,
Steve