I used PPG DDL-3053 Med. Grey Poly tinted with black to match some NOS parts that are also painted this color. I recall getting this a long while back from a Mustang Monthly article, possibly Bob Perkin's column. I've passed this along to others and all have been happy with the results.
This paint is used on the grille, rocker molding panels, and sport wheelcovers.
If your rocker moldings use the cast-in "Mach 1" letters, the surface of the letters should be painted bright silver. I applied "One Shot" enamel (found in art supply stores) with a rubber roller. This matched an NOS panel exactly.
See attached photo - note this is a metallic paint. It looks considerably lighter in the sun than in the shade.