Personally I hope it isn't just green fire because a color change really isn't all that amazing. Not to mention I'd prefer a dark purple to green. The warlock class has far more purple in its spells then green, and purple in WoW is just as much a "Warlock" color as green is. There is more to Demonology and Warlocks then Fel lets not limit our selves over something that has been hyped and glamorized for no real reason.
Not to mention if you have to pay gold to go back, do something difficult, or farm for an item it would just be silly. Because then you limit people from doing lore or playing around with something for no reason. If it is going to be a reward it should be that, it shouldn't be a life sentence. (Life Sentence being a figure of speech).