My plant is changing color.. ?

My green friend is at about 15 weeks. i put her into 12/12 at about 8 weeks. after 12/12 she started growing quickly, and then slowed down pretty good. it didn't get too much taller, a little fatter though. shes turning purple.. why?? is it temperature? it's usually between 60 and 80 during the day. at night its moderately controlled, but it gets kinda cold.

i want to know if she's in danger. if i need to change something i need to know what

Everything sounds normal. it will shoot up super fast and then stop and the buds will fill out. Don't worry! Its most likely temperature but not really a huge Danger, but I would get a heater of some kind just to keep the temperature a little more controlled.