Here'* something that might help: Pontiac engine colors
Wait, nevermind. Found this:
You want Ditzler(PPG) #15159.
On the other hand, the block and head may never have been painted at all. My grandparents had a 79 or 80 Concord, and it was bare metal, with a silver painted valve cover. That was a 258 though, AMC may have realized it was a bad idea to paint a motor that owners would be hitting with a sledgehammer after having to change the distributor cap every time it rained.