My plant always get purplish colored stems, I think it is a normal reaction to the strong light. There is another type of purplish stripes that can indicate a deficiency or overdose, but it shows striping that is very pronounced. My Sativa strains seem to do this more than Indica plants. Your leaves look pretty blue-green... is this a characteristic of the strain? Some nutrient imbalances (too much P or K I think) can cause a dark blue-green coloration, but unless the entire crop is overdosed, I think it is probably genetic. What strain is it, or is it just the lights causing the color shift?