Until the early 1990s, people in France drove with yellow headlights. But why? I know of two explanations. One attributes the yellow light to a rally in Monte Carlo on a foggy day. One team wanted to reduce backdazzle from the headlamps by covering them with newspaper. The newspaper reportedly was yellow and visibility improved. And in fact, the rest of the world did drive with yellow fog lamps for a long time.
The other reason relates to Germany: in the event of war, France wanted to be able to distinguish its cars from German ones.
Whatever the reason, since the early 1990s, the French have only been permitted to use yellow lamps in vintage cars. The light they emit was never really popular, because the colored filter swallows up to 20 percent of the output. What’s more, the beneficial effect in fog turned out to be an optical illusion.