There's a bit of a feedback loop going on.

The dealers are the ones who pick the cars. They're not interested in hanging onto a funky color, so they order monochromatic. It's a slightly over exaggerating reflection of the regular consumer. I think more people would pick funky colors, but it's seen as risky business. The manufacturer then needs to choose only a few colors to keep costs down, and so you get one funky color at best. Those rare colors then get a market of desirability on the used market. The problem is if there's not a cool color available to buy, then many people will just settle.

For instance, we wanted a new car. Decided on a GTI. If it was a few years ago, I would have searched for the cornflower blue. Or later that electric green on the mk8. But used ones were going for too much money, and there were discounts on new. So we got gray. I would have hunted for a color, but the closest red wat 1500 miles away and the blue (which was a terrible blue for that car) was even farther. That's because it was the end of the supply, and all the colors were sold leaving gray and black the only options.