It can be both the strain and environmental factors. Usually it is a cold swing of more than 10 degrees between day and night temps. Also, lack of phosphorus can cause some purple as well. The plants are looking a little nutrient deficient but they also look like the perfect flush. If you are more than three weeks from harvest, maybe hit them with an NPK ratio that is higher on the P & K to finish off the cycle. If less than three weeks, might as well let them fade out naturally.