First grow

Can be genetics, can also be caused by lower temps - night or day, but more likely at night obviously. anything at or below 68F has the potential to bring out fall colors. Your plants are clearly underfed and you want to flush them? Stop following bro-science and start reading and reacting the the plant and adjusting your methosd for healthier, happier plants deeper into flower for better results. With less photosynthesis possible all you get is slower growth/ripening. It's like poking a hole in your car's gas tank and expecting it to increase the range you can drive.

The colour bepends on the strain,some wont show any under idea conditions.

You still have 3-4 weeks to go. Your plants are hungry for potassium. The purple colour is normal and not related to anything. The purple colour is just an accumulation of pigments, a bit like Fall leaves.