Weren't there already some threads similar to this one?
Now, for me, it'd have to be white taillights. No, they don't work on every single car, as a matter of fact, they don't work for any of cars that weren't intended to have them out of the factory. (I'm thinking about Altezza-style lights on non-Altezzas here. Those just don't work at all.) But I still find it interesting how almost every single car on the market nowadays have white rear and side turn indicators, yet very few cars have white taillights. I know it's probably for safety reasons, but then, they don't have to be all-white, just not all-red as majority of the cars have them today.
The Lancia Thesis' back is a perfect example of how they can work. As a matter of fact, I find Thesis' taillights amongst the best looking taillights on any of the modern cars. They're minimalist and elegant, and they work for the car.
Another good example would be a 2nd- or a 3rd-gen Prius. They fit in the whole futuristic design language of the car.