Deficiency or genetic?

I'll try to add to what some have already said; When you spot an issue like this it's real easy to consider it a problem or a deficiency. In a lot of cases (especially on this platform) people tend to over diagnose issues that aren't really issues at all. If you're happy with the growth rate of your plant and she's not showing any other signs of a problem like droopy leaves, rust spotting, general "pissoffedness" etc. then odds are good it's just the nature of the strain itself. Judging by the photos, this girls got no issues with how you've treated her. That being said and what's interesting about this is your night temps. My grow cabinets carry similar night temps (and sometimes even slightly lower) and I allow for this on purpose. I do tend see a lot of purple in the stems of many different strains that I've grown and believe it to be for this reason as well. I wouldn't however suggest that it's a bad thing. Like any plant, cannabis will thrive in an environment of slight variability. You never want to have the medium constantly wet and you never want to have it dry as a bone for days at a time either. Rather, you want to have intermittent cycles of both to simulate a natural rainfall over the course of time. The same applies to temps. Just like with watering, a variable temp range (within norms of course) is actually beneficial to development and almost a type of stress measure to trigger certain responses. In summary - keep do'n what you be do'n. Colour is just a colour in this case😎👍