If your plants are healthy in living soil, then yes, it would most likely be due to temp drops. Certainly do not discount Phosphorus defeciency out of the question, but I think your organic method has left you with the proper living enviornment in which your plants can thrive without, so considering the time of year, temps, and the fact they should be finishing up anyway, I think they are just starting to autumn off and show you they are done. I just started to flush my outdoor girls (although I feed them frequently), and they just started to go purple on me, but we have also been having some 20 degree plus temp drops at night so I feel it is a combination of the flush & temps. If you feel you have atleast 3 weeks left in your cycle, you can consider feeding them bloom boosters & carbohydrates as a means to boost their size/potency at the end and prevent defeciency if that is the case. Or, maybe go with an organic tea & organic molasses so that you do not destroy the living enviornment you currently have.
+rep? What? Huh?
haha
Cheers