You can buy em off ebay. I think its a hot roddy thing. Ive seen some of the JDM boys put em on headlights.

They are a blue coloured plastic disc that from a distance they show the taillight up as being purple, red + blue = purple

In the states there called blue dots and make a difference when you hit the brakes mainly for show.

We have used Blue Dots in hot rods for decades. They are blue to look at, but give a lovely purple glow when you hit the brakes. Best seen at night.