So before the mud starts getting thrown around...

Some basic physical misconceptions running around here:

- Pigments and spectral colors are different. Mix paint pigments, you get black or brown muck. Mix equal powers of spectral colors and you get white.

- The WAVELENGTH (spectral color) that appears brightest to the human eye is GREEN. Right in the middle of the visible spectrum. Go figure. Green laser pointers look much brighter yet actually put out less optical power than red.

- The sky is blue not because of absorption. Blue is not abosrbed by the atmosphere appreciably more than red. Blue is Rayleigh scattered in all directions by molecules more efficently (a function of wavelength^-4), causing it to separate out from the rest and scattering all around, making the sky blue. Larger particles cause Mie scattering, which is directional and not strongly wavelength dependent- causing the sky to look whiter closer to the sun when the sun is high. At sunset, the sun's light travels through much more of the atmosphere to reach you, causing much more Rayleigh scattering of the blue, and also significant green scattering- leaving red. This is enhanced when there is a lot of dust or aerosols in the atmosphere, like after several days of high pressure and low wind, or during large fires.

- In rain, blue scatters as before and is compounded by the fact that it refracts inside raindrops more severely than red so it is bent out of the path you want to illuminate. Raindrops themselves actually cause very little scattering.

- In fog, the droplets are much smaller but still larger than the wavelength of light, and Mie scattering dominates. So white light still looks white, red looks red red and blue still looks blue when it gets to its target. But there's less of it because of the scattering. Lots less- that lost light has been bounced in the other directions- which is why you can see the beam in fog. And some of that has been scattered right back into your eyes, degrading your vision. That's why low beams are used, and fog lights are aimed low- to reflect off the ground and less into your eye.

- Yellow foglamps: Mie scattering, as I said, is not very dependent on color for the size droplets present in fog. Any color will penetrate with the same efficiency, so it comes down to finding the easiest, "brightest" color to generate. A single color is preferred to reduce image blurring caused by the high color dispersion through water. Yellow is used mostly for historical reasons because its close to the color of lamps anyway and could be filtered without too much loss of brightness (think about when they first came out, only incandescent lamps were available). Its also perceived brightly by the human eye, very close to green. Red/green couldn't be used (stoplights, emergency vehicles), and blue still has the Rayleigh issue. So yellow it was, and continues today.

- The 'colors' of HID lights are in fact from the projector system in the lens causing dispersion- the color you see changes with viewing angle. They are arclamps, which do 'burn' hotter than tungsten filaments, so there is more blue spectral content to begin with as well.