If I recall correctly, yellow light also works better in fog and rain. Since fog is water, it reflects blue light (which is why water looks blue). That light reflecting back means it is not penetrating...actually makes the fog appear thicker since it's reflecting off the surface. Yellow light passes through better without reflection. That's why some German manufacturers like it for fog lamps. Looks like electricalconnection.com had a yellow lens for our bikes at some point. I don't see it on their site now, but they do have aftermarket yellow fog lamps for other bikes...maybe they can help?

Oddly, the latest trend in headlamps is "super white" which often has a blue tinge...which works great in dry weather, but not so much in the wet.