Well my temps are low 70's with lights on and in the evening id say 67 maybe even a little colder. Is that cold enough to affect the color of the stems? I think the purple stems look cool, too bad it doesn't mean some purple ! haha
And i read about N deficiency which said it can cause red and purple stems, But i don't think that is it cause my leafs are normal, there isn't yellow. And growth isn't slowed at all.