Purpling on leaves

is the pH off? easy to rule that out... why not just a P deficiency? how much ppm do you give of P? if soilless try to be up around 60 ppm, assuming your fertigate every time with 10% runoff. with soil you probably feed at a bit higher concentration, once the soil is sapped of its nutes, but also don't fertilize every irrigation, i assume. i'm not sure what "other" is on your substrate info from diary. if soilless, knowing ppms or weighted-average (relative to ratio of fertilizer mix) of the percents off the labels of each product used gives a good perspective and makes for effective, informed decisions when problems arise. you can see high/low threshold based on what you observe with plant -- assumes pH is okay and not nute lockout. it has limits but if in the right ballpark to start, it's a very short learning curve from there. Even with soil, knowing such things are good once the soil is tapped and you fertilization is providing nearly 100% of the building blocks for the plant's growth (nutes). once you have smooth sailing, no need to track that stuff in detail, bt still good to keep track of what you fed and how often... when things do arise, when you have things nailed down, they happen slowly.. you may have to look back 6-7-8+ weeks before you see something on the leaves. plus, learning transition to bloom growth takes a little trial and error too, but same process employed. obvserve, react, take notes, do better next time.

High ph will cause lock out and a variety if issues. Too cold on roots will show purple in some cases. Also lack of nutrition can case these fading symptoms of color morphing