I can't believe the discussion in the CD is of this low quality.
Purple or magenta coloring shows up in plants due to anthocyanin which is a specific plant pigment. It shows up for several reasons, none of which are mutually exclusive. Cold as someone mentioned, but more specifically nutrient lock-out and therefore stress which kills off chlorophyll so the purple pigmentation shows up. If a plant is purple at the end it has had that pigment the entire time you can just see it now because the green is gone. With white pistils however there is less pigmentation and the white is simple water filled cells. As they die the coloration of the pistil shows. It has no bearing on potency and only a little bearing on what type of creatures will pollinate that plant. Bees tend to find purple more attractive while orange will attract more butterflies. Maybe specific strains have adapted to that in their region, but the weed we all smoke(d) comes from a very limited gene pool so that theory is highly unlikely, but then again the lovely cannabis plant is a tetraploid so there are plenty of wonderful aberrations which is what makes each plant so lovely. That is also why I'm pro-life when it comes to weed. Give every weed a chance to grow. Amen.
Peace,
Pimpy
EDIT: Okay so anthocyanin is produced once chlorophyll breaks down. Carotenoids are always present though.