I've had very potent purple outdoor smoke before. It was 100% indica and sent me in a 2hr couchlock, but I have also had nasty purple weed. One of the worst I've come accross.

If you grow a plant from start to finish in temps. under 70degF, the leaves will be purple. It is a sign a slight stress though. It is due to the lack of flow of nutes to the leaves. Nothing major, but I've seen it done.