dark stems

I’m currently growing grape smash strain Ph at 5.5 to 6.8 Ppm 1500 to 2100 1000w led full spectrum panel Growing in coco Been recently seeing my plant leafs and stem turning dark I was told the plant flower is a deep purple to almost black but I don’t think this is the cause of my problems...

Hi this is my first year growing indoors I just started flower on my plant but have been being too see darken stems and dark leaves on my plant any answers to what’s happening Love to upload some pic but have no clue how too