Hmmmm. I’ve always figured that the Dark One is called the ONE for a reason, because there aren’t multiples. Though if Rumple was teaching Zelena magic at one point, perhaps she tried to steal the power from him somehow without killing him, and a little bit of it did leak over, just enough to turn her green, but not enough to turn her greeny-gold-full-on-DO like Rumple.
Maybe she’s not a killer, and maybe it was that fact that she couldn’t bring herself to kill him in the name of power and instead tried to find a work around to get the power, that convinced him that she wasn’t actually the woman for the Curse job, because the Curse job was gonna require killing a loved one for power. We saw him test Regina’s resolve in killing because it’s what he needed her for, so I could see him ditching Zelena if she proved to not be able to kill.
So if Zelena has always wanted Rumple’s power, but has never been able to claim it because she just can’t bring herself to kill someone, it would make sense that she’s making him follow her around like a puppy while she waves his dagger around. It’s the closest she can get to actually having his powers for herself.
Sidenote: Her patchy makeup in that pic is leaving a bit to be desired. Hope they touch it up for the actual show.