The factory "default" color in '48 for GMC was Brewster Green; a very dark green. There is no color code number stamped or otherwise recorded for your GMC anywhere on the truck. The easiest way to tell the original color is by scraping/sanding slowly away on the bottom of the firewall where any repaints likely were not too thorough. Also under moldings and trim such as the headlight buckets, rear directionals (if you have them) just inside the door frames, and, the door trim around the window.
The interior color is matched under the dome light base where that never saw sunlight and didn't fade; most likely will match to 1986 Corvette Bronze Metallic which is what most GMC interiors using the dark bronze color are.
