Lead paint. The only way to get these vibrant colours that last a long time is by adding various Pb compounds.
In the late 80s an Early 90s, they started using other chemistries, which is why the vehicles from that period had such terrible fading and flaking.
They finally got it right in the 2010s, but by that time, people had settled on monochromatic or boring colours.
When you do see colours, they are usually Navy Blue, Jungle Green or Burgundy.