In 2012 Sterling Grey was discontinued and was not resumed in that model year, in fact it was not listed on order sheets at dealerships but the 498 cars in that model year were sold through dealerships. You are correct it did resume in in the 13-14 years.
There is a thread somewhere online..let me go hunting.
EDIT: Found it. Looks like I was off a bit, there are 568 Sterling Grey 5.0 coupes and it was not listed on the build website as an available color. I'm not sure if we are allowed to post links to other forums, but if you google 2012 Stering Grey Metallic Mustang Production Numbers you'll find a couple of threads.