I reached out to a seller on eBay and asked what vinyl they were using. Their response was Scotchcal 50. I looked that up and found that it's a cheap quality vinyl that isn't designed for long term outside use. Based on what I was reading, the 220 series, high performance vinyl, is what should be used for vehicle graphics. It looks like the SC-50 is ok for cars, but it isn't high quality.
A friend of mine has silver stripes on his Camaro and the surface started looking blotchy in color. I think he has the cheaper vinyl because a local shop put them on to cover some paint damage from the factory. I also had bowtie decal from GM on the hood of my Monte Carlo that was perfect. One day a kid backed into the car and the hood had to be replaced. The factory decal was no longer available, so the dealer's body shop had a local shop create one. The same blotchy color and fading happened in less than 1 year exposed to the elements.