Red allows your pupils to fully dilate for better natural night vision. Lights with shorter wavelengths (orange - blue) will cause the pupils to contract.
Blue is supposedly good for following a blood trail. Contrast with the red I suppose.
Green is the light your eyes are most sensitive to and Night Vision Equip't is less sensitive to green/blue.
Yellow is nice because while it's fairly bright, it's softer if your eyes are night adjusted. Sorta like twilight. It will still affect your night vision.
White is best when you aren't concerned with natural night vision and want to see the full color spectrum.
For my applications, I prefer white or red in a light I'll use a lot. The yellow Photon on my keyring was an experiement for me and the jury is still out.
There's an
article by Doug Kniffen that discusses the green vs red issue. The important to note is that while green
may be better for retaining night vision for reading star charts, the light must be at very low intensity.
Ideally, you want to use only as bright a light, red or green, as is necessary to perform your chores and no more. However, if you have a brighter light than you actually need, a brighter green light will generally have a more negative effect than an equally bright red light. Green or blue-green has a greater capacity to adversely affect night vision because the eye is about 100 times more sensitive to these colors, so even moderately too bright light can have a serious deleterious effect.
I've got a Gerber Recon (white LED w/ red, green and blue filters), but it's not dim enough for the purposes mentioned. The green and blue seem much brighter because at the same intensity, the eye is more sensitive to the shorter wavelengths.