Even if Ford did not place a color in the brochure you could pay extra and get a non factory color. It cost $98 extra. In 71 & 72 if you got a non factory color on a model that normally have a urethane bumper you would get a chrome bumper due to not having the special paint for the rubber.
In 1973 you got an Argent front bumper if you got non factory color. Several members have non factory paint cars they all have a six digit DSO code and not even Marti can tell you the color Ford used he did not get the information for Special Paint & Trim cars.
They go all the way back to the beginning. Here is a couple pictures of a 1964 1/2 painted a 1940 Lincoln color and also has real leather seats. I lost my pics of my special paint car with computer crash. This car is in the MOM, Mustang Owner's Museum in Concord, N.C.. Glare on pic of seat, not allowed to open the cars in the museum unless owner is present and opens it. The first on in the Mustang registry for six digit cars was built for Henry Ford II and had full leather interior.
If you find a car with six digit DSO code something is not a factory option on the car.