[As it pertains to aquarium bulbs, the degrees Kelvin thing has turned more and more into a marketing term. Which is why, as Mark points out, it varies with the manufacturer. In terms of actual frequency(NM) and aquarium bulbs, it's just a spot on the spectral graph of things the human eye can see.
Think of it like a radio, and NM wavelengths are like stations. We can hear all kinds of stations on our radio, but some stations are stronger than others. Your bulb is like the radio, you can get all kinds of light out of it, but different stations come in better on different radios. Ok, the radio analogy may be awful, but maybe it helps get the point across.
By way of example, the Iwasaki 6500K 400W bulbs has as much in the blue spectrum as many 10K bulbs. However it is a 6500K bulb, and it definitely looks yellow. This is because along with alot of blue, it also has alot of rest of the visible spectrum, specifically yellows and greens.
Here's a chart derived by Sanjy Joshi, which kinda makes the point:
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You can also see that while the 20K and the Coralife 10K have a bit more blue than the Iwasaki, they have alot less of everything else in the PAR range.
Similarly, "actinic" PC bulbs do not appear blue because they have such a high degree of intensity in the blue spectrum, rather it's because they have less of everything else. Though I've not seen any PC bulb data, it wouldn't surprise me at all to find that the yellow Iwasaki had way more "blue" than these actinic bulbs. I'm not saying it would "look" more blue, just that it would be more intense within that wavelength.
My only point is that "actinic" bulbs are mostly an aesthetic thing, and if you are using them instead of 10K's cuz you think it's better for your corals, that may not really be the case at all.]
