Looking at the pictures I do have from Mustangs around the 9F196xxx period and the limited Cougars around the same time I offer the following since the poster lacks any original clues - next best choose at this point IMHO
Anyway looking through everything we got lucky and I happened to have to samples cut from cars from about that period.
Here is a scan of them. Right one is in much better condition since it was under the sound deadener
Looks like the fairly typical dark gray batch with a tilt towards blueish green. I've used a dark gray VW gray exterior color in the past and tinted it to blueish thing it would work in this car also with maybe a little white (very very little) but would have to see what the green did to the mix first
Hope this helps
Also notice on the sample on the right you will see the sound deadener which would have been sprayed on your car over the batch color. Typically looks to be three passes front to rear from the center of the car (worker standing under trans and walking down the drivelien tunnel) starting just behind the torque box area on the sides and behind the trans in the center. Driveline and exhaust was not installed on the car at this period from what we find. All stop or fade away before the rear end rise in the floor board typically