Yellow bulbs...

[Way2go]

Hard to know whether the yellowness of the light or the reduced light output (or both ) makes yellow foglights that little bit better.

The response of the human eye is highest in the green part of the spectrum so presumably skewing it off to one side by yellow bulbs reduces glare somewhat.........

[electrokid]

Yellow is the colour that we percieve to be the brightest - it's very close to green in the region of our greatest sensitivity. It might be just me - but it seems that yellow is a very clearly defined colour yet it occupies such a tiny part of the visible spectrum. The other thing I find interesting is that the light frequencies of the visible spectrum occupy (just less than) one octave.