I'm not sure they are. I don't think most people give two shits about the color of a vehicle, that's why most of them today are white or silver.

I think people care more than you think and it's mostly an effect of the limited options setting the tone/culture for most of society so when "exciting" colors randomly come out, nobody wants to stand out amongst the sea of silver/black/white. I bet if cars became more funky on average, people would exercise a lot more freedom in selecting wilder colors. An example of a culture of fun selection is snow sports. Colors rule the landscape, and yes sometimes for safety, but it's also because it highlights personality.