It makes sense why corals would grow best under under 6500k bulbs. Natural sunlight is nearest to 5500 -6000k. As for brown corals, nutrient levels (i.e., excess nitrate and phosphate levels etc) also play a big role in determining SPS color. I think I need to see a 12k or 14 k bulb in action before I change from 10ks. I prefer the natural, white look with just a touch of blue.