What color flashlight preserves night vision best when shooting at night?

Red light, because your low-light vision uses a pigment called rhodopsin. This breaks down in bright light, but less so in red. It takes about 45 minutes to regenerate fully, which is why there's that period of re-adjustment.

Using red similar to a darkroom "safe light" with black and white printing, because many photo papers were less sensitive to red/orange wavelengths than to blue. As I understand it, this is mostly a (happy?) coincidence, although once photo chemists had a choice, the convenience of a darkroom light which doesn't significantly disturb night-vision may have been a factor as well. Of course, with panchromatic paper, there's no "safe" wavelength.

If you're working in the field, as Michael Clark mentions, it's valuable to be familiar enough with your equipment that you don't need the light. Not just the camera controls, but your camera bags, lenses, and so on. Of course this comes most easily from practice. Next time you're in the field in the dark, make a conscious effort to navigate in available light.