People who want to preserve their night vision use a red light. The intensity of red light is such that it doesn't cause the light receptors in you eye to become overwhelmed. Pilots may use them during the flight (I'm not a pilot but I'll assume that vtjim knows what he's talking about), astronomers - both amatuer and professional - use them, military used to use them.

If it's really dark, you want to be able to read something and still want to see stuff - like stars- in the dark, you want to use a very low intensity red light.

And no, you shouldn't use one on the front of a bike because people will think you are a wrong way cyclist or they may not see the light until they have invalidated your third dimension.