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.
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.