It is common to see older European cars from the 1930's through the 1990's with yellow headlights. It is even more common to see cars of all generations, even present day, with yellow fog lamps. Is this purely a cosmetic feature or is there a practical application for this? The answer is that yellow headlights have a purpose of safety.
Yellow headlights are referred to primarily as "selective yellow". The purpose of having yellow headlights is to improve visibility at night by removing blue to violet colors from the wavelength spectrum. On the color spectrum yellow light has a longer wavelength than blue to violet colors. The shorter the wavelength the harder it is for the human eye to see. When looking at the wavelength chart to the left, you can see yellow to red colors are ideal for seeing at night. As a side note, this is why BMW uses redish / orange color for its instrument clusters. In inclement weather, having a blue or white light shining on rain, fog or snow can cause a glaring effect thus reducing visibility. Yellow light does not reflect off water as easily as blue thus it can allow the driver to see the road more clearly.
White headlights are now the standard for todays cars as they are brighter and have a wider spread of visibility on the road. Today it is common to see yellow fog lamps in production cars to improve safety during snowy and foggy conditions. Lexus is well known for having bright yellow fog lamps. It is also becoming trendy to put yellow film over early model BMW bright lights for cosmetic or throwback reasons, but a side benefit is in fact better visibility at night (although you have about a 12% loss in brightness). This yellow film can be purchased on eBay or Amazon for very cheap and can change the look of your vehicle as well as add functionality. The best part is the process is completely reversible if you do not like the look.
Now that we know the science behind why yellow light is better than blue or white in night time driving conditions let’s learn the history of yellow headlights are called “French” lights. In 1936 the French government passed a law stating all vehicles produced from 1937 onward must have yellow headlights. Although the main purpose was for safety, some say this was also a way for the French government to easily discern a French car from a foreign car while driving at night. This was important during WWII in the 1940’s. By 1993 France was forced by the European union to conform to their standards of safety, thus the going away of the yellow headlights in exchange for compliance of using white headlights. If a French car was first registered prior to 1993 you are able to still have yellow headlights.
Check out an Old Top Gear video from 1990 explaining the French headlights further.