PAR is "photosynthetically available radiation." In other words the amount of light a bulb produces that a plant is able to use for photosynthesis. Basically, the lower the K value, the highter the PAR. A 6.5K bulb has about 1.5-2 times as much PAR as a 10K and 3-4 times as much as a 20K bulb. The bad news is the lower the K value the less "pretty" the colors look to the human eye. So that means you have to use more bulbs and more electricity to get the same PAR if you are using a high K value bulbs. Most fluorescent bulbs are usually used as a combo with daylight and actinic bulbs to get some sort of combination. These are often sold as 50/50 type bulbs. Most folks that are using MH will combine them with an actinic fluorescent to achieve the same thing.

On top of all this, if that wasn't complicated enough, you have the issue of actinic vs. "true actinic." lights. Some bulbs that are sold as actinic are actually blue and peak at around 460 nm. True actinic is purple and at 420 nm. 420 nm is thought to result in increased pigment production in some corals. Since its close to UV, they produce these colored pigments as a protection from the UV, just like we produce melanin and tan as a result of exposure to UV. 420 nm basically fools the corals into thinking they are getting sunburned without exposing them and the fish to the negative effects of UV. That's the theory anyway.