Yellow spots and purple stems

What Papa said seems spot on.. .. But I see you are doing a coco grow. I am worry about the way you water your coco. It looks way too dry from here! Coco coir is a fantastic growing medium but a few simple rules needs to be followed for success. Coco perlite, (or pure coco, or pure perlite) is hydroponic. It cannot be treated like soil. Coco needs to stay moist all the time !! Coco from brick is very salty and naturally unbalance. It will eat up your CalMag and Phosphorus. To counter act this, coco should be rinsed and buffered with Calmag. Coco should have a constant pH and ec-ppm. Watering should be done at least once a day, always with nutrients, until you get 10-20% of runoff, that should get discard in a drain to waste set up, or could be returned to the reservoir in a flood and drain setup. Of course you can *cheat a little bit* without killing your plants ..(like sleeping at your gf-bf house and skipping a day!!) But a coco grow needs attention and daily watering!!