In what way is it controlling her? Zatanna's behavior changed from before, yes. But putting it down solely to mind affecting influence of violet light is disingenious. Paul's behavior before and after the resolution of Nabu's situation is at least as different as Zatanna's. And hers should differ more:That's the thing though, she's NOT entirely controlling it, it's controlling her. She doesn't even have the benefit of a proper ring, and as a result (from what I understand) the effects on her mind will be even more pronounced. She has far too much violet text.
But really, if that were the case, it'd be fine. The problem is he hasn't even TALKED to her about it, or even thought about it.
1) She's an actual teenager with all that implies
2) She was far more emotionally invested in the situation
3) She had and has to deal with more consequences than Paul
Taking all those into account, I don't actually see much, if any, effects of the violet light on her behavior.
Also, bear in mind that she's not using raw star saphire. She's using a magical staff made by best atlantean mages to filter star saphire's power and uses that to power her magic. Which is why all her magic is violet colored.