As the natural light (outdoors) shorten with each passing day. Darkness plays a key role in the last ditch efforts to get fertilized. Once a plant figures out that its almost time to die, they go thru a few cycles of last efforts. If the pistils fail, the plant moves on to the leaves, or trim leaves if you prefer. Your leaves will actually turn snow white...not with oldness or decay, yet with loads of mini node receptors. If you pick up a 60x jewelers loop eyepiece you can see what your plant is really doing, and time to harvest.
The plant when put into total darkness for 24-48 hours the plant goes into extreme mode ....SHE WANTS TO GET LAID! She then over stimulates and produces twice the resin.....all the way into the leaves. Lol. The pic shown in this reply is caked full of tri-chromes.
Also, cold weather, low temps can make your plant change purple. It has to do with the genes within the cbg's.