Going green...

The kelvin has a measurable effect on the plant growth, but as stated before it depends on the plants in particular. Not to mention that the kelvin rating does not denote any particular wavelength spikes that might have been put into the bulb via particular phosphors. It's more indicative of the appearance of the light to your eye. Plants don't care a bit. They are only concerned with available wavelengths of light to absorb and use. Once again, every plant looks for different light - hence they are all different colors and shades. Whatever color you see on a plant - well, if you recall from science class, is the light they are reflecting. In other words, the light they aren't using.

For example you can buy a home store bulb rated at 6500k but that doesn't mean it has spikes or peaks in particularly beneficial wavelengths for photosynthesis. Now whether this really matters all that much is wide open for debate; there are schools of thought that swear by specialty bulbs with things like trichromatic phosphors (causing peaks in three key areas of photosynthetic wavelength) and others who say they get just as good growth with plain old 6500k daylight bulbs from Home Despot.

There are other considerations such as CRI (color rendering index) lumens (the actual brightness of a bulb - watts only measure how much power they consume, not necessarily how much usable light they produce) and as said before, restrike and efficiency. For example many people have reported markedly increased growth upon switching from larger diameter (t8 and t12) bulbs in higher wattage to normal output t5 bulbs in lower wattages. I personally found that my plants took off and got a lot greener when i went with one of those Coralife T5 Aqualights as a replacement for a T12 shop light. I stepped down from having a total of 80w output from the original fixture to a total of only 56w on the T5 and yet growth and plant health actually increased. Saving space, saving money, and yielding better results. In fact I am so sold on T5 now that I am looking into a high output T5 setup.

One good practice that people do if they have several bulbs is to mix up the kelvin ratings to guarantee they are covering a lot of the spectrum. That way what one bulb might lack, another will make up for. If I were a real nut and had available to me a rainbow of T5 bulbs, I would have a set up where the first lights to come on were very low K and warm - like a sunrise. Then would come the cooler temperatures, so forth and so on until all of the temps from about 3k to 8k were illuminated, then I would have the cooler ones begin to turn off one by one until all that is left on is the first very warm bulb (sunset!) I'm a nut. But I will do this one day. And results will be reported.

Hope this confuses you more! Happy planting.