It's more than likely the cooler temps. You have to remember also, that if your area is having cool breezes/wind.. this will effect the plant in that manner. Usually turning parts of the plant a purple-ish color.
I just harvested a Afghani #1.. and most of her turned purple-ish/dark maroon reddish color. Beautiful colors IMO.. especially when you have the red/purple mixed in with the natural green pigmentation of the plant... it just looks beautiful.
Anyhow, I know that there isn't a purple pheno of Afghani #1, so I know it was due to cool night time temps.. the color never made it all the way to the buds, but the fan leafs and some of the smaller bud leafs turned a bit purple/red. It's really beautiful IMO.. just got done with air-drying the Afghani harvest for 3 days and I just finished putting the harvested buds into jars to begin the curing process. All I gotta do now is wait the initial 24 hours with lids on.. then check my RH values.. and go from there. A perfect cure every time..
peace.