... What's the reason for this (genuinely curious). I keep hearing contracting answers ...
MH during VEG followed by HPS during BLOOM best replicates what the sun does during an outdoor grow. With the sun higher in the sky during the summer, the colour temps are in the 6500K range. The sunlight has a more bluish tint during the months of June through to mid August. As the suns position in the sky drops moving from those summer months into autumn, the colour temps also begin to drop.
When I first started growing with 4' fluorescent tubes many years ago, I always used the higher colour temps for VEG - the cool white tubes with the slightly bluish tint which better replicated the summer sun. The higher blue content during VEG generated more bud sites with shorter internodes helping to keep the plants from getting too lanky.
When they went into flower, they went under the lower colour temps - the warm white tubes with a slightly yellowish tint which better replicated the autumn sun.
And although I no longer use those 4' fluorescent tubes, I still follow that practice of dropping the colour temps during flower. My VEG light is the HLG BSpec with the colour temps ~4100K and my flower lights are Vero COB's with the colour temps of 3000K.
