Not all strains can turn purple, purple strains turn purple via the acumulation of anthocyanin in the leaves and buds.
Here is an insert of an article posted in high times:
The Color Purple
Purple Cannabis is all the rage in the Nocal clubs and it is highly sought after by both consumers and producers, lets explore this phenomenon and learn some things about Cannabis and Color. Do we see any great demand for purple today? Not in normal things because they’re readily available but when it comes to Purple Pot it’s another story.
Cannabis floral clusters are basically green, but changes may take place later in the season, which alter the color to include various shades. The intense green of chlorophyll usually hides the color of accessory pigments, Chlorophyll starts to loose it’s vibrant green color late in the season and anthocyanin pigments also contained in the tissues are uncovered and the amazing colors are produced at this time. Purple, resulting from anthocyanin accumulation, is extremely common in Cannabis, this color modification is usually triggered by seasonal change, and also can be influenced indoor by a temperature drop of between 20-30 degrees.
This does not mean, however, that Purple is controlled by environment alone and it can be passed on genetically. For purple color to develop upon maturation, a strain must have the genetically controlled metabolic potential to make anthocyanin pigments and be responsiveness to environmental change such as cold nighttime temps. This means a strain can have the genetic potential to change color but if conditions never exist you may never know it. I have grown Jacks Cleaner for a long time and when I moved west and it found some dramatically cooler temps I got a nice maroon/ purple coloring at maturity.
There are other pigments that effect the color of mature cannabis for example Carotenoid is largely responsible for the yellow, orange, red, and brown colors in buds. I find this effect is much easier to accomplish in Organics and soil but in order to really call a strain Purple it needs to have more than colored outer leaves and the inner buds need to actually be Purple.
and i love it i find no matter what strain i grow outside some turn purp:hubba: im growing mostly indica hybrids that flower in 7-8 sometimes 9 weeks so im not sure if its a indica thing or what cause i have no experiance with sativas but thats all changing write now i got a few new mostly sativa strains going but those wont go outdoors due to the fact they will never finish i know one of the strains CM is supposed to go 16+ weeks
