Variety is collapsing.

Cars are becoming more of an appliance than they used to be? Not sure. My car is anything but an appliance, but it falls in one of the "boring" color categories in the above chart. YMMV, no pun intended ;)

my golden car

Yellow staying steady.

The picture of conformity.

the obvious trend is that cars have become much more plain in recent years (i suspect resale values are higher for cars with more plain colors).

does anyone have a good explanation for why green cars have all but disappeared?

according to this chart, they were more popular than white, grey, and black cars in 1995.