Board Thread:General Discussion/@comment-25355646-20140926123929

      It appears that if the Dark Curse is to be enacted, then the ingredients (plus the sacrifice of the heart of the thing you love most) must be mixed in two entirely different elements: fire and/or water. Regina used a bonfire, Pan used the well, and Snow used a vat filled with water that had a fire under it so it was boiling. And I think the element(s) used during the curse’s enactment, effects the colors of the smoke formed (the colors we’ve seen are purple, green and black). When Regina cast the curse the first time, smoke was already coming from the burning wood, so that’s where the black comes from. The green smoke seems to always come with the curse. There is also a magical purple smoke.

When Pan enacts the curse a second time in 3x11, there is just water from the well; so there is no natural black smoke or the magical purple kind. It is purely green. And in 3x19, after Snow enacts the curse a third time, the color of the spreading smoke was purple and green, but no black. Because of the fire beneath, the water was boiling. So I guess the presence of heat within the curse’s mixture causes the purple.

So in my opinion, the reasons of the colors are:

Green: Always there. Automatic.

Purple: Heat.

Black: Natural, from burning wood.  