Green was a hugely popular color among all the manufacturers of the day. And not just on muscle cars--it was painted on many, many daily drivers, cheap cars, etc. It was part of the whole "earth tones" trend that took over our homes as well.

The "Battleship gray" is definitely a fad--but I see it very rarely on the roads and almost always on higher-end models.

If anything "silver" of today is the "green" of back then. You can't swing a dead cat without hitting a silver/gray car these days.