The COMMON 42 WATT CFLs found at Lowes are Warm white, or 2700K for Flowering, they are the Red Spectrum. Mostly for BLOOMING or FLOWERING.
Kelvin is a measure of temperature, as in 6500K, 5100K or 2700K (this is the emitter temperature, not the actual temperature of the bulb itself), and it refers to the color of the bulb; 6500K being a blue light spectrum, and the 2700K being a red spectrum.
5100 is a blend of both. Plants like a predominately blue spectrum during their vegetative growth and like red spectrum light during flowering. Together the 6500K and 2700K provide a dual, or complete spectrum, suitable for all plants during all phases of growth.
You'll need a High kevin bulb for Vegging. a low kevin bulb for FLOWERING.
To the eye, you'll see a creamy colored bulb for warm white and a bright white colored bulb, called cool white, for VEGGing. It is almost a blueish white.