==> car in question is my 67 SJ Fastback in Sig <==
As I was painfully cleaning each and every wire that's visible in my main front lighting (and other stuff like regulator, horns, turn signals, etc) wire harness and re-wrapping it to boot, I noticed that the red headlight connectors were stained from years of being in contact with undercoating. I couldn't remove the stains. I decided I needed a vinyl/rubber/plastic paint to remedy things.
In a HD run I went to my favorite spray paint section (Rust-oleum) and bought a few reds whose cap was close to my headlight harness red plug.
I ended up with Rustoleum Colonial Red Satin 249082 as shown in the attached pic. This paint is plastic rated and the plug remained flexible after painting (and clear) without cracking.
Cleaning the harness plug is mandatory and use a real aggressive reducer (not paint thinner). Allow all of the reducer to outgas or put the plug under a heat lamp to dry it.
Tape off the wires and apply a really thin coat. It should take little paint. It covered my black stains with only a light coat. Light is better so that the paint doesn't crack on install.
The plug here was deemed to be a bit shiny (even being a satin), so a light mist coat of Rust-oleum "Matte Clear Enamel" 285093 toned it down a bit.
Great looking plugs are mine!