That gas filler neck overspray was not the original color of your truck. That was what it was repainted. There was no fuel tank or neck in the cab when it was painted in the factory.

Those necks were black originally.

The best place (IMHE - In my humble experience) to determine the original color of a truck is the lower firewall behind the engine in the bellhousing bubble. Using a razor scraper and a piece of tape, lay the tape down then gently lift the tape while scraping the paint away underneath to reveal the original color. 99% of all repainting does not sand the firewall down to bare metal. If they do, then you can try to see what's under the windshield rubber on the top edge where there's probably no rust. Other than that, look under the outside door trim and get a flashlight and peek into the doors for over spray. If all of those places are devoid of any clues, make up a story and stick to it.

http://www.oldgmctrucks.com