Hey there FoxFarm,
How old are your plants? More info would be a great help. A lot of times, when a plant is young it will have purple stems but it will go away. If it is an older plant and has been flowering for weeks and weeks, and then you get purple stems. I would look into this.
How much fox farm and how aften are you feeding? Are you flushing your plant every other week or so? What is the PH of the Nuts and water, and what is the PH of the Run off.. (the water that comes out the bottom of the plant. PPM on that is very helpful also.
How long have you been in flowering?