What is Correlated Color Temperature (CCT), and is it good for comparing grow lights?
When you get anything warm enough it will start to give off light, and the hotter it gets, the more energetic the light gets, shifting from the red end of the spectrum at about 1350 °F to blue when temperatures get closer to 17500 °F. This "glowing" when things get hot is known as a "blackbody radiator", even though it does not appear black.
Correlated color temperature is a measurement of the average hue of light as it appears to the human eye, expressed as the temperature (in Kelvins) something would need to be heated to glow at approximately the same color. We have a more in-depth discussion of what color temperature is (and isn't) here.
Correlated color temperature is useful whenever you're comparing different lights that look white to the human eye, and for things such as adjusting "white balance" in photographs, to make them look right to humans.
When comparing "white" grow lights (multi-spectral lights that appear white to the human eye), the color temperature can be useful as an indication of the relative balance of red light to blue light in the spectrum- the higher the color temperature, the more blue there is in the spectrum (compared to red). But if the light doesn't look roughly blue, white, yellow, orange or red to human eyes, color temperature doesn't apply-- no matter how hot you get something, it will never glow purple or green, so color temperatures for these colors don't exist.
So while correlated color temperature can be a useful comparison among grow lights that look white to humans, it isn't useful overall in comparing grow lights. The correlated color temperature definition is based on how humans perceive the light, not on how plants perceive it.