Most of the purple strains will turn pruple with only relatively cool nights (purple urkle, grand daddy purple, grape ape, etc. I have some Purps that is turning purple on the tips and spreading. Sort of natural. However I also have seeds outside with buds turning purple, but when I grew em inside they didnt change color. Outside its been as low as 50 F some nights so thats obviously because of temp. Im hoping I can get some of my outdoor Trainwreck to turn purple.