To be 100 percent honest, I don't think I have ever seen any cannabis flowers that really looked like what I would call "purple," even "purple" strains from a dispensary. I've seen photos where the flowers kind of have hints of purplish hues, but I assume they could have been manipulated.
Now, I don't doubt that in 20 or 30 years after Big Tobacco and Big Agriculture and Big GMO have gotten their tentacles into the industry, you'll probably be able to buy seeds or starter plants that are "guaranteed" to finish purple, gold, red, whatever color you fancy. But the Big Businesses have billions of dollars to play around with.