Keep your temps. below 70degF, and the purple will start to show.

It's actually a sign of stress. The cooler temps. slow down the flow of nutes getting to the leaves, so the leaves turn purple as a sign of stress. Perty colors, but that's about it.

I don't understand why someone would pay more due to the color of the purple buds. Who cares, as long as I get high, I don't care what color it is.