Purple stems

The purple petioles, or the purple stems on the fan leaves aren't as concerning as if it was the branches. I would just continue to do what you're doing. Purple stems normally mean a cal mag related issue. Your plants will want nitrogen the first 3 weeks of flower. Then they will want cal mag around week 3-4 and 5 for bud development. Calcium is building blocks for cell walls

If you have a good nutrient schedule, this could just be natural colouration and there are no issues, I see this very often. If this is the only symptom I wouldn't be concerned. When there is a deficiency, purple leaf stem (petioles) is a sign of phosphorus deficiency but I'd also look for other symptoms, dark green almost blue leaves or brown leaves and slow / stunted growth. If you have a few of these symptoms then correct with more phosphorus. In the veg stage you need 30ppm of Phos. In flower you need 120 ppm Phos. in hydroponic / coco grows. hope this helps.

petioles -- not nitpicking but makes a difference in symptoms whether it affects stems or leaf petioles. it's normal for indoor plants. something about the artificial lighting or something... Low phosphorous could cause this, supposedly, but you'd see other symptoms that would help confirm it, and those symptoms are not seen in this picture. This happens to nearly every plant i grow. The petioles more exposed to light show a greater effect.

It means nothing.