I got thermal nail polish for (myself for) Christmas and I’m utterly fascinated by it. Red is cold, opalescent is warm. I’m going to chip the hell out of it in very short notice but it would’ve been a constant red on my toenails.
I know I had hot wheels or matchbox cars that did this as a kid. Now I’m googling if it has been done on real cars. I guess it probably wouldn’t look as cool although I love seeing flow-vis so who knows.