From wiki:
Whenever you play online, your world tendencies will shift towards an invisible unpublished global server tendency setting. Because this will make it hard to keep pure black or white states if desired, it is strongly recommended to play in offline mode when you need to carefully control your world tendencies.
Anyway, stay offline if you need to keep it pure white/black.
The global tendency is usually towards white, but note pure, so you can use it to your advantage if you need to get your world from black to white.
As for invading, that will only shift your character tendency afaik.
I have heard that quitting your game in the Nexus will shift it when playing online, so I always quit in 2-1 at blacksmith ed's.
But it still shifted on my last playthrough so I couldn't get the istarelle, may have something to do with someone rating a comment I left.
