Quote Originally Posted by WhiteTux
The K is actually for Kelvin. It measures the temperature of the light emited from the bulb.. And, higher kelvin values mean less lumen output from the bulb.
You're partially correct. Color temperature (expressed in Kelvins) is just an engineering standard that compares the dominant spectrum of the bulb with that of an ideal black-body radiator. (You can read about it here.) However, a cfl or florescent bulb's efficiency is not directly related to its color (k). What you're referring to here is the bulb's luminous efficacy, which is the ration of lumens of light output to watts of power consumed.

With ideal black-body radiators radiating at the same power, efficacy does drop off with color (k), but cfl's and florescent bulbs are not black-body radiators, and different types of bulbs are more or less efficient at different colors. It is safe to assume that the lumens declared on the box the bulb comes in closely correlates to the dominant color.

To the OP's question, I agree that mixing 2700k and 6500k bulbs will produce a good healthy grow. There is no reason NOT to mix them.