I had the underside of my hood repainted and the body shop used a nonspecific low gloss black to repaint it. It was a perfect match. I'm guessing the GT Guys are the best source for an official paint code.
I painted one of my riveted pans with a roller while it was on the car using high temerature enamel. Good match to the stock color, and held up well for years.
From the factory it was applied thru the E coat process. The entire frame was E coated in black then the silver was painted in the interior and engine bay areas before assembly. When we replace or repair these belly pans we have then E coated to look OE again.
In a pinch you could have it painted gloss black and clear coated but it may not match the other side exactly.