Board Thread:General Discussion/@comment-13475854-20140317121612/@comment-2157876-20140319200832

I think she become green because she failed her magic training with Rumple.

In Season Two episode "We are both" Regina is powerless and visited Mr. Gold shop to retrieve the book she learnt magic from. Gold warned her the dangers of learning magic in a rush, and Regina claimed she needed a shortcut and that it didn't matter if she becomes green.

So Regina kind of implied that an inappropiate learning of magic might turn someone into something else or change its appearance.

So what if Rumple made Zelena his apprentice and when she failed she turned her skin green. If Rumple confronted Zelena and mentions Regina, might be another factor to turn herself green while failing her training, as you wrote, maybe her jealousy led to her failure.