With the nocturnals I always use red light that I leave on 24/7. Most of the tanks also have a normal white/blue daylight lighting that goes off at 9:00 PM. The reasoning for the red over another low/moon light LED is the understanding that red is close to invisible to all cephalopods and you are more likely to see them under red lighting than any other color. The reason I leave it on all night is that they can still detect it but that is as dark as it gets so they don't wait for for full darkness to come out. On some tanks I only light half the tank and have found that the young ones take up residency on the lit side but usually den on the dark side as adults (but still come to the lit side)
Blue light, on the other hand is a color that many cephs are sensative to (this may be for deep water animals and may not be for the ones we keep) and may be brighter than white light, discouraging a nocturnal.
This was something Edie Widder mentioned in her discusion of the eJelly and the Medusa vehicle used to attract the Giant Squid in last Sunday's Discovery documentary. We have had members try using blue but only
recently did one of our members experiment and report. He initial took my suggestion to set up a red light for his nocturnal macropus once the animal seemed adjusted, he tried a blue light to see if the animal acted differently. Great experiment and I encourage others to try different lighting in the same way BUT suggest you start with red.
I have found that a fairly bright red light works well for viewing (and miserably for photography) and have used different variations. The simplest, cheapest and easiest to set up is to
obtain a cone shaped (ugly) shop light with a hanging clip from your local hardware store. Also at most hardware stores and at Wal-mart, you will find
red screw in fluorescent bulbs (often called party lights). The shop light's cone shape keeps the light out of your eyes and focuses on the tank but the red fluorescent bulbs have a standard screw base and can be used in any fixture supporting a 13 watt bulb.
A more attractive alternate but dimmer and more expensive light is one of the programmable LED moon lights that offers a selection of colors. This is nice to have when you want a moon light on the tank and the next animal you have is not nocturnal. One feature I suggest looking for is a unit that will keep your setting when the power goes out. I used one for a diurnal octo but removed it from the tank because of frequent short outages that summer. The one I had defaulted to white light when first turned on and Octane would pace the tank under stress when we did not realize his light had changed. This would be my choice if it was brighter but the fluorescent gives far better viewing.
Lastly, any light that you can make red works. On the tank I used for my mercs, I had an outdoor light and put high temperature vellum (high temp material required or you will have a fire concern even though the light was a lower heat bulb) inside the lens. Later I painted the inside of the lens red with several coats of red paint. Both worked well but I preferred the light of the vellum over the paint. White LED's with a red lens is also an alternative and I use a dome light on one side of one my split tank but wiring them up is a bit of a pain.