Depends what you want to accomplish and how much lighting you want... If you just want to be able to see things un-disturbed going about their nightly business, go with red. If you want to get into simulating lunar cycles to try and promote spawning, use blue. If you just want one or two LEDs to make the tank look neat, use blue
Ok thanks. I have a hard time seeing my crab because he doesn't like the blue so I guess he'l like the red. What I think I will do then is use both, go light, blue, red, blue light to combine the light and moonllight with a dark period with the red to see the fish at night.
Many reef fish/inverts/crabs/etc cannot see red wavelengths because they dont encounter them ever on the reef... Red gets scrubbed out of the water so most organisms have not adapted to see it. Red light to them is like infrared radiation is to us. Its still there, we just can't see it