God, here I go with the science again. Forgive me. The purple is not the result of GMO, though that doesn't mean someone hasn't tried engineering. That trait has been described for a very long time, especially in strains from colder climates, ie Indicas. Purple in leaves comes from a class of pigments called anthocyanins. Anthocyanins do not participate in photosyntheses like Chlorophyll. They are primarily used by the plant to absorb harmful or useless wavelengths of light. Typically we don't see them because they are found in relatively small quantities, and the coloring from Chlorophylls swamps out their presence. But under certain types of stress, like nutrient or water deficiency, they may appear, because chlorophylls are being exhausted, usually to provide a source of Nitrogen and Magnesium. This is why leaves of hardwoods change color in the fall. The greens disappear as light and water fade, and you're left with purples, reds and yellows from all the "accessory" pigments, like anthocyanins and carotenoids. Anthocyanins belong to a larger class of biomolecules called flavonoids, which are powerful defense compounds. They protect the plant from oxidative stresses and UV damage. They are a good thing, and there is no need to engineer them in, they're already there. They just appear more frequently in strains and/or conditions where certain stresses are occurring. Frankly I'm concerned because my plants are entering week 8 of flowering and they're still all bright green!