as long as the bulbs are below 40 watts, you can also lightly colour them with spray paint (flat red)...hold the bulb about 16" away and mist the bulbs, let them dry for 24 hours. We do that all the time at the zoo...works just fine !
Alternativley there is someone selling LED set-ups in various colours on ebay at the mo so you could try them, think they also do underwater ones, prob not the best for a ceph tank thou, if you put them underwater.
But on the pic it looked like you get a fair bit of light off them..... :)
I think cold cathode tubes would work very well in this situation as well and they are readily availible at just about every pc shop. I use blue ones in my case mod. I'm not a red or green fan. It would be very easy to seal these for under water use as well as top mount. Any PC shop would have a used 200watt PSU lying around for free I'm sure. I do, if you need one. but this would give you the 12volt rail you need to run the cathode tube. This type of light will put off a flood style of lighting rather than a direct light of the LEDs. PM me if you want to explore this option further. I dig deep into PC's
I've read back through this thread and thought it might be a good idea to point out the reason we're talking about the color red for lamps or LEDs - octos cannot perceive red, so you watch a nocturnal octo without scaring it.