That's a good question.
I wonder if different factories used different colors, or did every plant use the same color?
I'll bet the concours guys would know this.

 

Not sure about the process in the 60s, but in the 90s we used different colored primers based on base color of the vehicle. My guess is that Ford did have at least 2 different colored primers. Likely grey and red.

 

I don't know about earlier, but by 1968 finished unibodies were dipped in an electrically charged big tub of red rustproofing primer, then sprayed with gray primer over that, then sanded and polished for any imperfections prior to paint. Any odd colors after the gray primer would have been painted color specific, like forward of the cowl and (sometimes) the outer rockers, but the underbodies were often some odd mix of whatever they had on hand and so colors were not always consistent there.