I've read & seen lot's of Pics about the "Purple " Buds. people like something exotic, different, etc. And Purple leaves & Buds is certainly novel. First things first. Some strains has the 'purple charateristics more than others. It's advertized & sold as such. As far as having a higher THC, that's BS. But People will pay more simply because it's different.
Here is what I accidently discovered. One week prior to harvesting, I give one final, big time watering, that's my flushing. The next day, I move the Lady into the House (which has AC). I'm "drying" the Lady on the Hoof/Vine (stalk). They may have to wait in line for a trim a week or so. Anyway, my Master Kush turned Purple !!!! A BEAUTIFUL Purple, even though they are not advertized as "Purple". It was the LOW temperture (70 to 72 degrees) that brought out the Purple charateristics in the Plant. So, if you Lady is into Purple, keep her cool for a few days, and see what happens.