Re: What does 6500K actually provide for plants?
My understanding from old reefing knowledge is that the color temp is mostly relevant to our color perception.
More relevant to photosynthetic response is what wavelengths make up that 6500K. A certain color temp can be the function of (1) a 6500K monochromatic light source (such as some LED chips), or (2) a combination of a wide range of wavelengths (such as the sun). The latter is likely preferable for the general care of plants, and it is preferable for our viewing pleasure (since a wide range of the colors of an object are reflected).
Looking at spectral curves of various light sources can be interesting and informative. The closer those output curves match the spectral response curves of the plant (or, for reefers, photosynthetic animal) you're trying to grow, the more of the provided light will be useful to that plant.
"People are like plants, they grow toward the light."
- Hope Jahren