leaves are turning yellow towards the end

This appears to be a natural part of the plant's aging process. If the leaves are turning yellow, drying out, and dying at this stage, she doesn’t necessarily need more nitrogen (N) fertilizer, she needs the right balance, depending on your specific growing conditions, and judging by how deep green the rest of the plant is, I’d say she’s already getting sufficient N. At this phase, her focus shifts to phosphorus and potassium (PK) while drawing stored nitrogen (a mobile nutrient) from the lower leaves. She won’t grow taller anymore; instead, she’ll direct all her energy into bud development and allow the older leaves to fade naturally, just as nature intended. Maintain a low but available supply of N (avoid overfeeding), while prioritizing higher PK levels. One last thing are you absolutely sure you’re providing enough phosphorus (P)? I ask because I use a high-potassium bud booster (NPK 13-0-46), and if I don’t compensate for the lack of phosphorus, I see the same purple stems you’re experiencing. A P deficiency can restrict bud development, so it’s worth double-checking your feeding regimen.