My bet is that it's Navajo Orange metallic. It's a 73 Pontiac color that was supposed to be only used on the smaller Firebird and Ventura, and not one of the colors normally offered on the A and B bodies in the 1973 "buyer's guide" dealer brochure. Since it's a 73 Pontiac color that was available at the time, it would make sense that it could be special ordered for use on a GTO. Buccaneer Red was another color that wasn't offered on the A-body in 73, but was also probably special ordered a number of times. Below is a PPG paint chart showing the Navajo Orange color.
73 PPG CHIP CHART by
grand73am, on Flickr