Here's my take. The correct color is in the eye of the beholder. Until one actually sees an original engine with most of the paint still present, one can only guess if what they choose is close to the same color.
I have seen two in original paint, including the original yellow Thrift-master script, and to my eyes, it resembles more of a lighter shade of black, than a dark gray. The best way I can describe it is that it is the color of oil-soaked road grime commonly found on these leaky engines. In other words, as you clean off the grime to get to the paint, its hard to tell if you did.
The discussion gets interesting when someone suggests that GM may have not used the same shade on all engines. I haven't seen any reference to a source for this hypothesis.
Here's my other take. Among all of the opinions flying around, one's chance to be deemed correct as to one's chosen engine color, is as good as if you asked your wife what she thinks about your opinion on any topic (in my house anyway).
Hope this helps.
