While, at least in my original 1967 or 1965-72 Master Parts Catalogs, I have no separate “Paint” Section, I do in my 1960-68 MPC. In the “Exterior Paint (Acrylic Enamel)” Section, is noted “Dark Gray Metallic, Code 5, Part Number AR60 1988 (Mustang tutone, lower back panel)". This color and code also shows up under "Exterior Paint - Acrylic Repair Lacquer".
What makes this interesting is that beginning in 1966, all Fords were painted with Acrylic Enamel (AE); yet the tutone Dark Gray Metallic is noted in the "Exterior Paint (Acrylic Enamel)” Section as “AR” (Acrylic Lacquer). It is my personal opinion that Ford, not wanting the tutone back panel to be "glossy" like the body paints, went to the Acrylic Lacquer, which in its unfinished, un-rubbed out, unbuffed state exhibited a "semi-gloss" or "semi-flat" appearance. I cannot think of another reason for this being done.
In order to get a better "idea" of what the Dark Gray Metallic noted looked like, I had my DuPont jobber mix up a pint. Of course they don't do Acrylic Lacquer anymore due to all of the Fed regulations, so using the Ford Code for the Acrylic Lacquer, they mixed it in Chroma Premier; to this I have to add a "flattner" in order to do away with the gloss look. A couple of years ago when a similar topic on VMF led to someone bringing up the color issue, a member replied that, to him, the SEM Dark Titanium Metallic, Part # 39263, looked very close to the original Ford Dark Gray Metallic, Code 5, AR60 1988. I obtained a spray can of the SEM, and painted two sample pieces in order to compare the colors. Not wanting to mix my high $ pint of mixed paint yet, I just brushed my sample right out of the can (no flattner), which I felt would be adequate for color comparison. As you can see by the attached pictures of the two samples, the SEM paint (at least to me) appears to have more black in it, and not as much metallic as the Ford Code mixed paint, which also appears to have more gray in it. Of course if you've decided to not use the tutone option, your grills will be the same as the color of your car, and you won't have to worry about the Dark Gray Metallic.
I have also attached a picture (which I believe originally came from Jeff) that shows what he meant about how the paint was done underneath the rear grills.
Hope this helps,
Bob