Simple answer:
Generally speaking:
If you want to have your corals to look what they should look in a shallow water environment, colorful, growing faster than ever and exploding with health you can give them a 6500K metal halide for Christmas and they will love you forever!
If you want them to get colorful and still with good growth and showing amazing health you can get a 10000K or a 14000K metal halide over them.
A 20000K metal halide will still give you good growth and color too, but not like the other ones. 20000K metal halide will still give you much better results than ANY other type of LED or T5.
Why halides? because you asked me the BEST and corals see "color" including UV and IR radiaton. I don't think we should even argue about that. No other artificial light normally found for aquariums will bit halides in that sense.
Yes, of course, corals need "white" light! Just look at the sun and you will understand. No arguments here too because God is the one who made all that anyways!
This guy......
Reef metal halides are not full spectrum and he was just in the zoanthid forum telling people not to use LEDs for zoas /palys which is hysterically bad advice. A 6500k metal halide just puts out more green and yellow / orange spikes than a deeper blue reef halide, and I can post the spectral charts to prove it.
Halide bulbs have narrower bandwidth spikes than LEDs by far - industry fact, not an opinion. Again, google the spectrums for various reef halides and you will see how much color spectrum is missing . A 5600k LED has a far more gradual visible spectrum than tubes or halides. All are missing big chunks of color compared to the sun or true full spectrum artificial light sources like plasma sulfur.
Metal halides do produce some levels of true UV, but this level varies to a large degree depending on filter and jacket types. No proof UV of any sort is required for coral growth. In vitro spectral absorption myths are commonly debunked when the entire macro organism is taken in to account.
'White' is not a color or wavelength. White is a composite of colors that to our eyes looks 'white' (covered in grade school science class). I can produce an off white with just orange and blue. A little higher CRI if I use red green and blue.
Not going to comment on the "IR radiaton" other than do you pronounce 1.21 jigawatts or gigawatts?
If I compare the spectrums of 5600k CCT halide, LED and flourescent light sources they will all have a similiar component of blue with the LEDs having a smidge of far red if it's phillips based LEDs (Crees tend to stop around 630nm) while the tubes and halide have some short spikes below 400nm. The amount of any UV is still trivial.
All 5600 kelvin artificial light sources otherwise have massive amounts of 450nm blue. This is basically the same age old argument if 20k lights grow coral better than 10k lights with just different marketing terms thrown in. Both grow coral well, with water chemistry being far more important. The most amazing SPS tanks I've seen in person are all using black boxes on basic lamp timers with two modes - off / on. If you can't grow coral with simple black boxes the problem exists between keyboard and chair (PEBKAC).