Hey đź‘‹
When the stems turn purple, it can either be related to the genetics or a sign of an impending nutrient lockout caused by too high salt content in your substrate. Since this symptom occurs simultaneously with the dying of your leaves, I don't think you have a nutrient deficiency, but rather that your soil pH is 7. Long story short: I would recommend checking the pH level and only then continuing with fertilizing.
I hope I could help. Happy growing 👊🍀