To us its not. One thing to note is that our eye's have adapted to be more sensitive toward the green end of the color specitrum than the blue. This is because we live on land. One of the reasons we use actinic lights on our tanks is because the blue end of the specitrum penitrates down into the water better than other colors. Thus corals and IMO
fish have developed a greater sensitivy toward this blue end of the spectirum.
On the other end of the spectirum is red and its been said that fish can not see red light. If this is true then you could use a red bulb on your tank at night BUT certin undesirable algaes that normally live toward teh surface of the water might enjoy that more than you want them to.
I avocate a day/night cycle. Fish are used to some light but only that which is provided by the moon at night. its alot darker under the water on a full moon than it is on the surface.