Have had a few conversations with the Z about this, but he is a pretty old codger now (LOL, this is intended to set him off!) but his recollection is a muddy mauve - gray - red last time we discussed it several years ago.

This was available in a Pactra craft enamel odds and ends color many years ago. It was fairly close to some pics I found at the time. But you would need to reach out to a national club tech director for a final answer.

The underside is red oxide, which can vary in tone. Xynolite used to sell a ratle can that was a dead match for the OE color, still working through mine. Very similar color for the third-member, too. I'm sure one of the Mustang resto sites would have a good clue for a mix color.

That said, the Galaxies had a lot of undercoating in the high-trim models such as LTD, XL and 7-Liter. Typically this was slobbered into areas where driveline noise was a problem, like wheel wells and transmission tunnels. IIRC, there was even a factory undercoat option, but you would need a real live 1966 worksheet to confirm that.