Yes dropping temps can help turn it purple. Its the leaves way of conserving its self and its energy when it gets to cold and theres not really enough light. I belive the purple helps take up certin spectrums of light when it gets to that point alsl but i could b wrong. And i think its chlorophile thats retreating back into the plant that leaves it purple. Also good chance im wrong again but im pretty sure thats whats goin on with cold temps and purple bud