Mine is a 1960 GMC and was built at the Oakland GMC plant. Best I can tell the bottom of the original hood did not appear to be black. From old photos and videotapes it seems to be more of a thin medium gray (perhaps primer) with yellow overspray around the edges. The original body color was Lunar Yellow. I don’t have much to offer in the way of photos:
And here’s the best shot I have. It’s a photograph of a 20 year old videotape the last day the engine was run before it was pulled out:
