The under body on my 08A car was a very dark gray back to the rear axle and a much lighter gray from the rear axle back. Car was a black car, and had a lot of black overspray on the lighter part of the underbody (even though the tard that painted my rear valance FAILED to cover all the way to the bottom of the valance from the factory).
I used a combo of Dupont black paints with varying degrees of white to replicate the colors that were there originally.
I used RM lacquer (with some flattener) to paint the black out on the firewall. That is the most durable paint on my car.
So, firewall black, first 2/3 of underbody very dark gray, back 1/3 of the underbody lighter gray with lots of black overspray.
Pics here:
https://www.yenko.net/forum/showthrea...updated%2Apics
My belief is that this practice (as well as stipe and "white out" treatment for Z's withe white stripes) varied greatly not only from one month to another but from one shift to another.