Silver Creek has it bang on re: night vision and colors to use.

Something else I'll throw out for everyone to chew on - red light does not penetrate water nearly as well as blue or green. Yet another reason to use red light while fishing. It is still probably a good idea to turn away from the area you are fishing if you have to turn on your light though. Once your eyes are properly night-adjusted, it does not take much red light at all to aid vision.

A small, red keychain 5mm LED will be more than sufficient. Make sure to use a red LED instead of a red filter over an incandescent bulb as the LED will have a pretty pure red wavelength with little spectrum overlap (there are several other reasons for this too).

The link over to the Candle Power Forums thread contains more information than most of you will ever need. It just so happens that I posted on that thread over there....