Why are my Purple Power not Purpling??

I got 2 6 foot purple power plants and they are budding all over. I got em feminized from marijuana seeds nl. They have reddish hairs all over em but i dont see any purple yet. They have about 3 weeks left. Do they start to purple at the end? Any info would be greatly appreciated.

I think the colder the climate the more they purple up. Also the closer to harvest they more they purple up too.