I'd go red. Oh, wait...
I was debating between the green, San Merino and Ruby Red when I ordered.
I saw the green on an M6 with what I think was amaro interior and it looked amazing. It was over ruled by Mrs B saying it looked a bit of an old man colour and thinking about it now, I'm not sure how long it would have been before the novelty had worn off.
I saw plenty of M6s in San Merino and in a way that's what put me off. I wanted something rare.
I saw a 4GC in Ruby Red and was sold!
Based upon your other choices, it seems to come down to personal preference. I would say the purple was too showy and would attract every boy racer to have a go at the traffic light Grand Prix. The grey isn't showy enough for what it would cost and a grey car isn't a rare sight. Green, obviously an old man colour choice
so Ruby Red it is
