Here's an interesting article about Mustang underside colors. Apparently the red oxide primer was mixed 50/50 with whatever extra paint Ford had at the time so there is not one true color. Anything between brown and pink should work.
I would suggest that the old article was not specific enough and did not reflect San Jose cars. (Dearborn started using the batch colors for undercarriages in 65) Like most articles, especially older ones, most of the information reflects how Dearborn cars were assembled.
The article is too general but we have learned allot with the help of the internet. Unlike what is stated in the article red oxide could be the common or least common color depending on year and plant.
Got to be careful IMHO is using old magazine/information as we uncover new info every day.
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As for the "right" color - yes you need to start with an epoxy primer - sealer to get the correct smooth surface (not the flat open one we sometimes see on refinished cars) Its best to custom mix that coating to match what was found on the car being painted. The color at San Jose in 68 varied a little from a darker muddy color to a pasty somewhat pinker finish.
If you don't have a good example left from your car then search out a use the color found on another San Jose car built at the same time. Its the next best thing.
While saying this remember that the firewall forward (including the bottom of the front frame rails) was painted by had with a red oxide but often a different tone/tint. This reduced waste and is often more of a true red oxide color than the floors/undercarriage.
The red oxide was generally used in the engine compartment while a light gray was used on the exterior body panels
Hope this helps