Underside can be either red oxide, black, or a blend of different colors of primers. Red oxide primer is the most likely color. Body color overspray should be on the floorpans, tapering off as you get closer to the tunnel.
65 Mustang Convertible 289 C-code
Ivy Green with Ivy Gold & White Deluxe Interior
I think we need to know where your Mustang was built to give a fair answer. Metuchen used mostly red oxide, Dearborn I belive where the one most likely to reuse different paints mixed together when they primed the undercarriage. Not sure for the San Jose cars...
I think you should be able to find some spot on your undercarriage that has the original color. Who knows it might be something flashy, like purple metallic ;-)