You are going to need some good ventilation in there in the summer. You will not be able to keep going as is when it hits 130 in your attic. I think you will also need a small heater for during lights out in the cold parts of winter. You don't want a temp shift between day and night of more than 15 degrees. Plants will not die from cold unless they freeze. They can get stressed and appear to almost stop growing from it, and the colder it is the slower they grow. Your plants are too small to start tying down at this point. Wait about another 2 weeks probably. You can encourage elongation by pulling your light back farther. Also blue light encourages bushy growth while red light encourages taller spindly growth. Purple color means nothing unless it is on all of them. Many strains are purplish. If it is on all or most of them than it is a nutrient problem, but not sure what. I've often had purple stems and parts of leaves and buds on my AK-47, but not so much on my blueberry x northern lights (but a little).

Good luck. Looking pretty good so far.