if growth is all you are looking for, go for 5600K. not much to look at, since the bulb is so yellow, but that mimics sunlight and would give best growth. most reefers are first concerned with looks, then growth rate. the minimal difference in growth you would get from 10K to 12K isn't worth worrying about. the best color i've seen (and this is totally personal preference) are the 14K. i don't remember which brand it was, but it looked like the perfect marriage of white light with actinic, all in one bulb.