I think the build plates were only used up until 84. 1984 was also the first year for the SPID sticker on the center console door.
So the GTA would not have a build plate on the radiator support. The info would be on the SPID label in the center console for the GTA.
I think you are safe calling it "Brown". That is what it looks like to me. Pontiac just chose to give it a fancy name

If you need to get it painted just use the color code and the official name and you should be fine.
I grabbed the name out of the 84 Pontiac Sales book.