Factory correct is just gray primer and body color over spray along the outside edges. But most everyone paint the whole underbody with body color for better protection.
Main floorpan was grey primer/sealer with exterior paint color overspray fading in from the sides. Wheelwells are body color with masked undercoating in unique patterns with slight variances as it was done by hand.
Google “Comeback Cuda” to see a lot of photos for a restored Cuda convertible by Mopar Connection magazine (Kevin Shaw).