Weed strains having purple genetics tend to resemble purple color naturally, thanks to their intrinsic anthocyanin production. Another way to turn your cannabis plant purple is by providing night temperatures more often. Nighttime temperatures force the weed plant to produce more Anthocyanins and less Chlorophyll. However, it may not work with all the strains. Also, this process is not recommended as some cannabis plants tend to produce lower yields and slow growth when gripped on chilly nights. The minimum feasible temperature should be around 15 Celsius (59 Fahrenheit). Get an excellent indoor grow tent kit that can contain and sustain the temperature you set.