leaves seem a bit dark green have cut down nitroge...

Your leaves are beautiful, they look absolutely fine to me. Since you don't give a PPM or EC amount, it's hard to say if you are possibly overfeeding, although 15 ml/L of nutrients seems potentially excessive. The purple stems look to me like a natural, healthy type of purple. They've been purple from the very beginning it looks like. As a plant goes deeper into flower, colors can deepen or change. We also don't know from where/who this strain comes from. If it's just bag seed, or seed from a plant which you crossed, it's quite likely the purple is simply due to genetics. If the purple stems are actually unhealthy, it's most likely due to OVERfeeding or nutrient lockout, not that there isn't enough phosphorus in the soil. Get a device which can measure the EC/PPM values of water, and then you can check to see what the EC value was of the solution you've been giving them (by mixing up some new solution). Additionally you can measure the EC value of water runoff from the container, to get an idea of how much salts are in your soil.