Porsche got back to me on the code 99 for my 1978 Turbo 930, it took awhile.
Turns out 291 color code is for a special color from GMC for a 1978 Buick. The owner ordered it from a GMC PPG color chart called Medium Green Poly, so I would say it's one of one in this color.
It's code #45 on the GMC color chart attached.
