[pogodzib]

Each brand of bulb rates their k-value slightly different and the quality of the bulbs can very greatly. In general, the higher the k-value, the more blue and less intense it will be. A 6.5k bulb is yellow, 10k is white, 12k has a hint of blue, 20k very blue. Usually, people with lighting that is weaker will use lower k value bulbs to gain lighting intensity and coral growth rate. People with lots of lights over the tank don't need to worry about intensity as much so they choose a higher k bulb to give the tank a more blue look which helps to make the corals pop out. 10-12k bulbs are the most popular because the white/slightly blue hue and coral growth rate.