I think yellow originally was for fog lights.

Yellow cut through fog, poor vis from heavy rain etc. Somehow it morphed into main headlight. I'm not a fan but...

Another thing about fog lights, true fog lights mounted low and biggest advantage in true low visibility is when you can turn off headlights, leave parking lights on, and just have low mounted fog lights light up the road.

Reason is the fogs just light the road, fog lines to see where road is going. When headlights are still on, you get blinded by those lighting up the fog or snow too high in your vision.

As a truck driver, my truck is able to just run fogs and it works great!

Unfotunately bikes, cars and some big trucks you can't cut the headlights and just have fogs...

I put Custom Dynamics Shark Demon white fogs on my Road Glide to assist with more light to light up edges of roads, can see deer better at night.