Yellow headlights used to be a legal requirement for driving in France and we had to put yellow film on when driving there until the EU standardized white headlights.

Apparently it was to reduce dazzle and glare: https://en.wikipedia.org/wiki/Selective_yellow

As far as I remember that didn't happen in all countries. I remember seeing them when going on vacation to France (Corsica) in the mid 80s, but never at home.

No, it was only France, where they were mandatory.

In other EU countries they were however allowed up to a certain date (in Italy they are accepted only for vehicles built before 1993, and in the same date France made them not mandatory).

The yellow is not "a" yellow it is a very specific one, called "Selective Yellow":

https://en.wikipedia.org/wiki/Selective_yellow

I have no idea but could be related to use of the color yellow for caution / hazard lights, construction vehicles or turning lights.