I've seen much purple bud in the past, it's a natural effect in your Mj plant when it goes on low temp (pretty sure already been told), but it works better on certain strains and when big temp drops occur; You can read good info here; http://marijuanamythbusters.com/2011/05/10/chronic... ~ Happy Smokin ~
Color change doesn't affect the bud's potency, no worries about that, just makes your weed look quite tempting/cooler Without accounting the strain your plant is; the color really stands well for the classic ''purple kush'' hehe