I think it's CRI - color rendering index. Natural light (sun) has lots of light in the various colors. CRI of natural light is 100. Surprisingly incandescent bulbs are also ~100 (probably since they are blackbody radiators, same as the sun). CRI isn't the same as light "color".
Artificial light isn't as well spread out. CRIs can drop into the 80s and lower. Really low for the old "fish tank bulbs".
You can still grow plants (good PAR), might still look good to the eye (color temp), but might be missing some of the spectrum.
A mix of bulb colors (LED or otherwise) is what lots of people do to help spread the spectrum out.
I've got a 50/50 of warm white and cool white 10W LEDs (8 of each). I'm toying with swapping some out for some blues and/or reds as it looks a little warm for my tastes.
The RGB LEDs look interesting too - red, green and blue in one unit. Needs it's own ballast however.