Since you are growing outdoors, the purple leaves are a natural process that happens due to cooler night temperatures.
You have nothing to worry about due to the purple leaves.
Senescence does not cause purple leaves, rather it will show as yellowing leaves.
The purple is absolutely due to cool night temps, nothing else.
I would say that you still have about 3 weeks to go, maybe even a bit longer.
Once the plant has 98-99% brown pistils/hairs, you will be entering the harvest zone.