A good area to get a perspective on the underside color of your car from the factory is above the gas tank on the trunk floor. I cut a section of the old trunk floor out before sending it out for bodywork to match my mix up to. I used PPG DP90 and DP40LF to get the right hue. The 90 is black and the 40 is green hue. I mixed them to a gray and then added a little more DP40 to shift it a bit more green. In the shade or dark it looks gray in the sun you can barely see a green'ish hue. The other are correct that the majority of colors sprayed in the day or week before your car determines the color shift from black to gray or other variation. I also have never seen an original car to have red oxide under. The earlier 60's models did. San Jose built cars all had red oxide. It is my understanding Dearborn and Metuchen production shifted to slop earlier than 71.
Interesting. The area above my gas tank was the body colour, in my case Pastel Blue. I suspect that the primer colour was underneath this body colour, but both the underside of the trunk area and the adjoining frame rails ( side facing the gas tank) were body colour (heavy overspray). Another area where it was dependent on the assembly line painter and where and how much paint they applied. Cannot stress enough the need to scrape away the crud in various areas of the undercarriage to determine factory applications, for those looking to return the car to near factory appearance.