Its because rusts, golds, greens and browns were the popular main stream colors across all design in the late 60s and early 70s. Those years also happen the best years for Mopar Muscle and Chrysler painted cars in colors that they knew would sell. Most people therefore remember Mopars as being green, brown or gold. Along the fringes were the fans of the brighter and bolder psychedelic colors and Mopar offered the HIP bright colors to draw those customers too.
