Lumens is completely different from color temperature. Lumens is how much light the bulb gives off, judged on a scale based on the sensitivity of the human eye (which is actually the opposite of the sensitivity of plants for photosynthesis, but that's a whole different conversation).

Color temperature (kelvins) is how "hot" the light is, based on a rough approximation of a blackbody curve (if you don't know what a blackbody is, don't worry about it). Basically, lights with low color temperatures are reddish and referred to as "soft" or "warm", while lights with high color temperatures are bluish and referred to as "cool", "bright", or "daylight". For seedlings, you want high color temperatures. For mature plants, you want low color temperatures.

No, you don't need one of each; that's a myth. In fact, one of each is virtually identical, spectrum-wise, to simply getting bulbs of moderate kelvin temperature. For seedlings, you want more blue in your mix, while for mature plants, you want more red. Blue encourages short, stocky, leafy growth. Red encourages tall growth, flowering, and fruiting, and is generally a little bit more efficient (although it varies from species to species).

In short: go with the highest color temperature you can get with the highest wattage/lumen rating you can get for the most efficient type of bulb you can buy, and make sure those plants are positively flooded with light. If it's fluorescent, keep your bulbs right up close to your plants.

thank you so much KarenReI. You've explained it very clearly.

I think there might be an exception to the higher K is better statement if you go into the 10K plus range. Some aquariums use the ultra high color temps because it shines deeper in the water, but it might have less of what plants need. I have some 6700K bulbs on the way and I probably wouldn't go any higher unless I saw something convincing to the contrary.

I think there might be an exception to the higher K is better statement if you go into the 10K plus range

{{gwi:1025029}} {{gwi:1025030}}. Compare that to

Now, sure, if you get too high, you're getting into UV, and you don't want to be hitting your plants with that. ;)