Yellow leafs, purple stems

Purple stems and yellow leafs can be indicative of either magnesium (calmag to the rescue) deficiency which needs to be corrected quickly or can also be brought on by phosphorus deficiency. Phosphorus interacts with Calcium in plants, yellow leaves not typically found in early stage Phosphorus deficiency but if it comes in tandem with another deficiency or a pH problem it can yellow leaves. The plants are looking pale all over too from looking at the diary, might be N hungry as it is flipping in to full on flower - if you are top dressing nutrients mix with some fresh bacterial innoculant to encourage a quick breakdown of the organic matter.