That doesn't really make sense to me. You want to encourage the LIVING plant to produce its maximum purple pigment, and then at the end of flower, to encourage it to use up all its green pigment (chlorophyll) which often masks pure, clear purple coloring in healthy plants.
'Round these parts, the outdoor folks will sometimes let their crops take a frost. Personally, I'd be hesitant to do that, because of the potential for physical damage, but it will certainly help to bring out pruple coloring in genetically predisposed plants.