Lava red replaced grey when the 2012 Mustang production began. In the spring of 2011 the pigment for that paint became unavailable due to the Tsunami in Japan. Ford continued to produce/fill Lava red orders for the next few months, then they discontinued Lava red and replaced it with Grey. I believe the only other time Lava red was offered was in a special Harley edition of an F series truck a few years earlier.
From time to time I would jump on the web and Google "Lava red convertible" just to see what pops up, which by chance is what I did tonight and I found your post.
I too own one of 96, 2012 Lava Red GT convertibles produced. Attached is a page from a thread I found last year with the build numbers, they are pretty rare. I don't know what the numbers in parentheses on that sheet are, but if you take 2.36% of 4066 GT convertibles you get 96 lava red ones. Total Lava red production is another story, there were lots of hard tops and V6 convertibles produced. Of 88,064 - 2012 cars, it looks like 6869 other configurations were also painted lava red.