Purple Stems, Redish / Purple leaves

Hi all, hoping someone can help me out, my ladies are having an issue with stems turning purplish in color, leaves seem to be yellowing somewhat and im getting reddish purplish patches on some of the bigger fan leaves. Im growing in pro-mix and using general hydroponics maxigrow at about 2/3rds strength. I usually feed at between PH 6-6.5, my runnoff PH is in the 6.3 range. This started happening with my last grow aswell, slow growth, purple stems and yellowing that seems to start all over the plant (not just the bottom or top) I have flushed numerous times in an attempt to regulate the problem with no luck. temps are 23 celcius. I have no clue what the prob could be. Any help would be appreciated.

I like to share this photo with new members. I don't use general hydroponic nutrients on my grows so can't tell you if that's part of the issue. I just make soil with worm castings, guano with soil and coco tek. How far into the grow did it start happening?
From what i've seen low temps can cause red/purple colors. Can't recall what the optimum temp is for cannabis anymore... damn old age.

To me it looks as though you are deficient in cal/mag and Nitrogen for starters. Yellow tips would suggest over feeding but N deficiency looks like an N overabundance. Pro mix has no nutritional value and if you are feeding less than recommended I would say that's your problem. Purple/red stems are not necessarily indicative of a phosphorus deficiency, however those purple leaf veins are. Do not flush! Feed, water and be well. Don't starve them or they will react negatively as you have seen. Of course these are my thoughts only.

Its worth a try, I'll pick up some cal mag tomorrow and throw some into the mix, hopefully that will help, tnx again.