theres many variables to this. usually strains that turn purple have certain indica traits, or even plants that are bred with a known strain that has the purple trait. the key to this is to have lower temperatures during the flowering period. the lower temperatures usually bring those purple traits out. nirvana blue mystic is just one of these. greenhouse seeds the church is another one ive seen turn purple. theres also a product by humboldt county's own-purple maxx that will also bring these colors out. i have seen brick mids from northern regions be purple too. its usually harder to do this indoors unless you have a serious cooling system, which is why youll see strains that are actually called "purple" this/that being classified as an outdoor strain. basically if youre an indoor grower and you have normal 75-85 degree temperatures youre buying the strain name and the other qualities in the strain. you may get a purple color but its few and far inbetween. hope this helped you a little.
