Hey there so the red stems can be many things. Genetics, purple strains, berry strains, common. Phosphorus deficiency and magnesium too, but you have a good nutrient line up so unless you'rehaving lockout or ph problems I'd say not nutrient related. The 1st thing i saw was your lower humidity which can also play a role just as much as high/low heat can. Your plant seems overall healthy though, so i wouldn't stress too much...but of coarse finding out the source of our problems is always good. My guess is either the genetics or slightly humidity temp based...i think anything under 70 degrees will start initiating color changes. Happy harvesting ✌️