I am not well versed on the subject, but my understanding is cold nights do it. It all depend if your strain of cannabis has anthocyanins. Some strains, or better to say phenols, have more present, and don’t need the cold.

One cool example I saw, was some wedding cake I was getting from a buddy who works at a greenhouse. Some wonderful bud! But it was light green all summer. But the batch grown in the winter(light dep), was the darkest purple!! I will have to say I liked the buzz the purple cake gave me.