Like said before red is ultimate for not impairing your nighttime vision.
I have a buddy that goes to a maritime academy and they are required to have a flashlight that either has a red filter or red led's. They require it because of the effect that red lights will have on your eyes (not making your pupils get smaller) so that they don't have to adjust while on ship at night.
Ever since he told me that I've been buying headlamps that only have red led's in them.