Some fish don't see red light very well -- to them, red light and no light are pretty much the same. If they happen to be shy nocturnal fish, red light is a good way to get to watch them at their most active. That's what its mostly used for.
Other fish see red just fine. Mine slow down a bit, but remain active under their red moonlight. When the tank is completely dark they just seem to hang in the water, maybe adjusting position occasionally. Each lighting scenario has a very different activity level.
I haven't tried blue yet, although I've been meaning to. (I tried a shop called Just Bulbs earlier this evening on my way home (any Letterman fans?) -- struck out again.) The point of the blue is to more closely mimic moonlight in a naturalistic way, but the red isn't darkroom red -- its a warm dim light that I actually like.
I have a red 13w CF on my 30g (compared to a modest 72w daytime lights). I think the 13, in my case, is a little too much. I'd aim for less and maybe a more even spread if (when) I do it again. At such very low light levels I don't think there will be much photosynthesizing going on.
