Wondering why the top half of my new stems are red...

Hi mate! well this could be a lot, but it seems like cal and mag defficiency. Not a huge problem and easy to solve, just try to use some cal-mag, use good ph'ed water and you will make it easy. Some plants actually grow like that, deppending on the strain . But no worries, everything looks nice ;) check my grow if you want :) And be free to ask any questions you have!

That’s usually a phosphorous deficiency.

There's a couple of reasons that could happen. Low temperatures is one, and although your temperatures could be slightly higher, I don't think that's it. This happens to me all the time and in my case is because my tap water is too soft and lacks calcium and magnesium (Mg deficiency will give you purple stems early on). I would advise getting a EC/PPM meter and check your water source, ideally it should be around 0,4-0.5 EC / 200 -250 PPM, if it is way below that, you know that it's lack of mg. Then get a Cal-Mg additive and use it with your watering.