You have to take the cars out of the formula and look at the times overall to get a feel for what was driving the color choices.
It had been proven over and over again that the more distressed the financial situation is, the darker the colors being chosen on cars. The greens, golds, and blues tend to be in the upper end of the list, which is interspersed with the occasional high impact selection. Overall, consumer confidence was fair with the occasional chemically induced "wahoo" effect that made a few bright colors seem nice. Compare this to later in the 70s when cars were coming out dark brown, dark green, and black and overlay that with run away inflation, oil embargos, and stagnant economic growth.