Technically speaking, since the eyes are in scotopic vision in the light levels you usually have in low light with headlights (probably closer to mesoscopic), your eyes are more sensitive to light closer to the red end of the spectrum, therefore it takes less of that light to see more detail. Since you have less ability to discriminate colors in low light anyways (since your cone cells are non-functional in low light) you don't need full-spectrum light. Higher wavelength light that is reflected will get your cones working more and your rods less, so your night vision is reduced. Coincidentally, this is why it is stupid to put HIDs (even yellow ones) in fog lights.
Of course, this yellow color and the effects on your eyes from reflection are only important in high glare conditions such as bad weather or fog. Under clear conditions, very little light will be reflected back against you, so full-spectrum light is better, since you will be able to see contrasts between different color objects easier, even if you cannot see the colors. If you aren't throwing off light in a certain wavelength, an object (such as a deer) can easily become invisible against certain colors. This coincidentally is why I hate HIDs. They don't give you full spectrum, only selected bits of the spectrum to simulate full spectrum.
There is also some thought that selective yellow light will glare less when refracted in fog or through moisture in the air, but, I am starting to lean towards the school of thought that that is untrue.
Any other color of light is useless for you if you want them so you can see. They are only useful so others can see you, which is why we have red taillights on cars, since our eyes are most sensitive to that color under low light, so we can see other cars from a long way away without them having to produce a lot of red light. Think about it, your tails are visible from as far away as your headlights, but they have 1/20th the lumen output. Lights higher on the color spectrum, like blues and purples, while pretty, are almost always useless from a visibility point of view. Under low light, it takes more lumens of blue light to see that blue light than it does something like red.