It may not turn purple. Grow it and smoke it when it's done. Don't worry about the color.
If you want guaranteed purple weed get a pack of ACE Pakistani Chitral Kush. If you want a real Purple Haze get a pack of ACE Oldtimers Haze and hope for a purple pheno. But be prepared for a minimum 16 weeks flowering time. Most of the stuff out there people are calling Purple Haze are hybrids and not anything related to the old Purple Haze made famous by Jimi Hendrix.