Cold does have a little to do with it but also just having a room that does not get to hot.
I set mines to 75 in the day and 65 at night but genetics is the main thing. This one turns purple outside through the inside to the stem.
Also light must hit the buds to ripen them properly I noticed. The buds that do not get much light are not as dark but my plant naturally lays down from the weight of the buds and the sides get dark for me.
Some of the last few pages I seen, them plants turn dark early on and mines don't till week 5.
Just the tips turn colors while everything stays green till a week before ripening.
Here are a few snaps from last night.
They should turn 95% dark in week.half time.
Here is a cross section shot of one I broke on the way out of the room.