It is a choice of consumers. If it is cheaper to make boring colors, and consumers actually care enough about color to pay extra for it, then manufacturers would sell more brightly colored cars.

But they don’t. A car buyer goes to the lot and says “I want a car like x, y, z, and I want it to be bright red!” The dealer doesn’t have red, because most people don’t want red, so they say “okay, we can have that car for you in two months. Or you can drive off the lot today in this black car for $X less.” The buyer realizes they don’t actually care that much, and buys the black car.

The buyer can always then go to a paint shop and change the color. But they probably won’t do this either, since, again, they don’t care that much. Sure, they’d like a red car - but then they’d be without it for a few days while the painting was done and it would cost a bunch of money.