What temp turns a plant purple?

[TheShastaCoMan]

Now for a valid answer.....below 50 in the roots.

exactly, i was going to post that also.

if you have a plant thats prone to purpling in cooler temps and your growing in a climate that doesnt get that cold, you can take a bag of ice and pour some in the pot during lights out. also reduce the daytime temps in your grow room to below 55 degrees. in a couple of days youll notice your plants turning, and once they start they dont stop.