purple strains

Depends on the strain but most won't turn purple until about halfway through flowering. Mine usually won't show their colors until they start yellowing up at the end of flowering

I let my temp drop about 10 degrees for lights off during the first 4-5 weeks, then tend to increase it gradually to a 20 degree drop by week 6-harvest. Once they start flushing to either yellow or purple they tend to do so fairly rapidly in my grow.