It's all in the genetics. it has absolutly nothing to do with the temps or anything else. here is a plant i harvested last week......
This isn't exactly true. While genetics certainly play a major role, you can induce purple coloration on strains that are not predisposed to it. One thing to clarify about the temperature that people keep getting wrong here; it's your NIGHT temps that will bring out these pigments. If your night time temps are 15 degrees cooler than daytime, with a max of 65 degrees, you can bring out the purple pigments in many strains. I've even done this with Super Lemon Haze to good effect. This can also be accomplished (though I don't recommend) by inducing a phosphorus deficiency.
Having said that, there are many strains that are naturally purple and don't need any environmental factors to bring it out. There are also many strains that will not turn purple no matter how hard you try.