I don't see any purple stems, bro.
But it can be from cold too. And genetics. And pH as you said.
It is in the early pic/s. For sure pH lock out or lack of whatever nute it is that causes that to happen. I got plants in near freezing, green as can be. I never 'care' for them, take them inside, never watered them, anything, so, naturally they are my healthiest plants. Soon as I touch a plant, the thing is messed up lately.
I am "KISS" grower. Don't bother with pH, humidity, temperature etc... 2 times I defoliate them, once weekly I put new water in drum and collect runoff, and that's all I do. Everything else (feeding - which is my only automation) is on timer. In other words, I don't babysit them. Nor overcomplicate. They'll grow just fine.
And I'm with you there, trying to be anyways. Keeping it simple, is the way to go. I just set my fans low enough so you can't hear them, temps and humidity will be fine inside a house, plants are a lot tougher than me when it comes to that stuff and inside a house is a cake walk for them.
Getting my setup, setup, has been terrible at best and for no real reason. Half of my problem was using poor pH scale. Going back to my old ways, when stuff grew well. The other half was just finding the right rope to use for the watering. Of course, I was nearly tripping over the rope the whole month-month and a half I was trying to figure what rope I should use.
Aside from that, trying to get my setup to where I only have to pH water in a res.. 1 gallon pots are full with organic soil that 'has nutes' for the whole grow. Rope connected to the bottom of the pots, connected to res below , the res does all the watering automatic, at the plants digression, never have to do anything. All of my defoliating lately has not been for the plants growing to well, unfortunately.
Hope soon I can be with you on plants growing just fine and actually keeping it simple. I think I'm almost there. Enjoy.