How did the body shop match the paint? Do they have a hand held scanner that samples the pain on the car? They should. Its a small device that reads the reflective values of the paint. Because no two cars are the same color.
Btw... I have never heard that SGM is hard to match/paint. I have heard that of the "Cashmere Silver"(Gold-ish) color though! Silver is also hard to match.
I think the painter just mixed up(no pun intended! lol) the weights of the paint mix - before he sprayed your paint. Get them to re-do their work! You bought a grey car not a GREEN one!!