From my decades-long 3 years of experience…
I’ve red that read (HA!) is an indication of Phosphorus unavailability (not necessarily deficiency!). It also has something to do with “anthocyanins” and is essentially the same as fall tree leaves color change, and relates to the changing light spectrum that naturally occurs with seasonal solar decline/incline. At the same time the temperatures(esp. nighttime) are cooling down.
My winter & early spring plants were almost all reddened & purpling except one. Zero red or purple, all light green on the “runt”. Now, in summer it’s clone shows a bit of reddening. wtf. should I be a scientist and re-veg it & make clones & run them at all different temps & spectrums? Sorry, heheh, no can do.
So it seems it’s genetics first, environment second when it comes to leaf color expression as well as all our other favorite sticky traits.
I imagine that pH throught the plant itself varies a surprising amount and is also determined by genes then environment.