How Cold Is Too Cold?

I'm trying to make sure my plants are in a cold environment to bring out the purple colors, but at what temperature will stunt their growth?

From what ive read a plant stops growing if its under 50 degrees F. so as long as your above that they will grow!

My temps range from 66-81ish, they should be fine

I personally don't let mine go below 60 degrees.... But then again Im not going for the purple color either....

I could already see some purple on some of the fan leaves before I topped them, so I'm anxious to see how much purple I get when they start growing more