I started without using a heater I was getting low 60's in my 24/0 period, amazing what a difference it made putting the heater in there, plants grew fatter and faster. Now temps outside are down in the 20's and plants are getting close to an end so I don't know if purpling/darkening leaves are from the genetics or cold (on a strain known as Tangerine).
Use a heater of some sort, I got an electric heater with digital timer and thermostat for around 40 bucks, it's 28 degrees out my room with lights on stays around 72, I always measure temp furthest from my heat source. My room with lights off, well the heater runs up there so it's probably staying around 68 (furthest plant from heat source)