Why you are using yellow color as a head light in vehicle?

From the mid 1930s to the early 1990s, France required all headlights, driving lights, and fog lights to produce yellow light rather than white. This was originally done at the request of the military, which wanted to be able to identify French-registered cars after dark. Later, French engineers and scientists determined that the special yellow color they used, called "selective yellow", reduced glare from car lights. The requirement for yellow lights in France was dropped in 1993 because it was out of line with pan-European standards that called for white headlights. Other countries that permit yellow headlights in the past (some of them still do) include Japan, New Zealand, Brazil, and various former French colonies and protectorates.

Modern studies show us that compared to white light of equal intensity, selective yellow light can reduce glare and improve bad-weather seeing. The French situation did not really fulfill the "equal intensity" condition, though; the headlamps in use during the yellow-light period were relatively primitive and their output was rather low. Adding a yellow filter to the bulb or lens glass reduced their output by about 10% to 15% versus an unfiltered white light. So, a chunk of the glare reduction experienced in France was simply due to lower intensity reaching drivers' eyes.

There is more detail in two articles linked below.