Kelvin describes the overall colour temperature of a light (think of a piece of iron being heated up, the hotter it gets the whiter it gets, starting out red. Hence the high numbers being whiter.
It's an estimation of the overall look and you'll need to look at the spectrogram (the colourful picture bit) to see where the bulb peaks to tell much about it's plant growth potential.
As is stands, most of our lights make predominantly blue and red light, which are the colours that chlorophyll prefers. Humans tend to view bluer lights as brighter, even if the total power being put out by the lights is lower. This can be useful, as it means that we accept weaker lights, which keeps algae down.
14,000K tubes will grow plants, but generally they predominate in the higher end spectrums, losing some slight where the plants really want it, so we tend to suggest not going much past 10,000K tubes. 14,000's will grow plants, just not as well.
The next issue is the green. Plants don't use a huge amount of green light, it tends to be reflected, which means that green light will make the plants stand out better, but is not so good for growing.
As an example, here's a tropical tube
and here's a 14,000K
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