I believe that certain speciality lights such as GroLux can not be reasonably described by using a Kelvins number.
In physics, if you stick an object in a vacuum and heat it up, it emits light along a black body radiation curve. From this curve, you can determine its temperature in Kelvins. Kelvins is a temperature where the degrees size is the same as that as Celsius, but zero Kelvins is where molecular motion stops. See
http://en.wikipedia.org/wiki/Color_temperature
Our sun has a spectral radiation curve which makes to use a Kelvins number to describe the light. Artificail lighting devices that try to emulate this light, at least to the human eye, can be described with a Kelvins number.
However, fluorescent tubes produce light via phosphors which do not act like black body radiators. Our eyes have red, blue and green cones, so we can trick our brains into thinking that a light is the same as a daylight by using red, blue and green phosphors. However, the spectrum from the tube isn't a smooth curve, but a bunch of spikes that average to the human eye/brain to be similar to a smooth Kelvins curve.
But this is not at all the case with a plant bulb. GroLux and the equivilant Zoomed tubes produce a bunch of deep reds and blues because that is what is most useful to plants. There is no game playing to trick our eye/brain into thinking it is a white light, so it's ridiculous to give it a realistic Kelvins number.
