For a fish-only tank, you only need to worry about what looks good to you unless you want to grow pink coraline algae.
Your best bet is to visit an LFS, find a tank that has the color light you want, and ask them to show you what they are running.
Most LFS's use either 6500K or 10,000K bulbs, both of which recreate a noon-day sun color, except the 10,000K is notably more blue and less yellow (it is supposed to simulate noonday sun at a certain depth of water).
Here's my quick-n-dirty on lighting.
4200K--- cool white bulbs like you see in schools and grocery stores.
6500K--- "daylight" bulbs like you can buy at Lowes for T12 flourescents. Looks great on freshwater plant tanks, and higher-output bulb types (T5 and metal halide) will grow coral just fine in a reef tank, but they look pretty yellow. Corals can turn brown under them, but that doesn't imply that they are unhealthy--- just brown.
10,000K--- still sometimes called a "daylight" bulb but significantly bluer. Very common by itself on a fish-only tank. Some 10,000K bulbs look more "white" and some, like the UVL Aquasun 10,000K can have a distinctly pink hue to them. I have no idea why.
12,000K to 15,000K----- About the same as 10,000K just more blue. The UVL 12,000K bulb is also called a "50/50" and IMO it looks like neither! It's still pink. I have one, and actually like it, but it doesn't look anything like 50/50 to me.
20,000K--- a term usually used for Metal Halide bulbs, which basically means "really blue", but not as blue as the bulbs below. Lots of folks use 20,000K MH bulbs alone with no supplemental actinic lighting, and they tend to look great.
Actinic 420nm (nanometer)----- this is the old "03 Actinic" that was once touted as the most beneficial light source for corals, but it has been shown by Grim Reefer to be (usually) pretty low in outright PAR output. Basically these violet-colored lights are one step up from a black light. They make the corals' colors pop like NOTHING else can! They look great, but for a fish-only tank, not the best option.
Actinic 460nm---- like the ATI Blue+----- a very, very intense blue bulb that really makes colors for anything blue pop, and if run in tandem with something like a 6500K, 10,000K, or 12,000K, you can get some very nice colors. This particular color is one of the most photosynthetically useful lights because it peaks just at the point that the corals can utilize the most.
Now, all that being said, if you have a T5 flourescent or Power Compact light system, I personally recommend that you run one 460nm blue light and one daylight bulb (either a 6500 or 10,000K).
I personally like the look of the 6500, as a lot of fish have some beautiful yellows that really jump when seen under 6500K lighting.
If you have T5, I would use one ATI Blue+ with one GE 6500K. Another more "blue" option would be to replace the 6500K with the ATI Aquablue 12,000K.
If you have Power Compacts, you can get the 460nm blues and daylight bulbs from
www.ahsupply.com
