The Class is Greener on the other Flame
Truly, the fel fire is something that should have been in the game for a long time, but I am not sure if Blizzard had decided that demonic fire was green until they began developing Burning Crusade, and by then the warlock class and their normal red fire was already in game.
The ability to get green fire as a warlock will be a hard thing, requiring a drop in game that then takes you through a chain that even highly skilled and well geared warlocks on the PTR have said is very hard to do. Once you complete the chain, which takes you into the Black Temple, Mr. or Ms. Warlock Hotstuff will be casting any fel fire spells as green. It’s a lot of work for a simple change of color, but I am sure more than a few warlocks out there will be very happy to have the change.
According to Rho, the point that he might want to disable green fire would be if green fire became ‘ultra-common’, which is to say if practically every warlock in the game had the green fire ability. This concern is offset by the apparent difficulty of doing the quest, added to the rarity of finding the item to start the chain in the world. Between these two it does sound unlikely that green fire will become ultra-common any time soon.
Druids cannot change their moonfire pink, nor can Priests change their shadowfiend into a rainbow monstrosity of cheerfulness. I personally don’t think Blizzard should have made warlock green fire quest，it should have been that all warlocks would be moved over to green fire spells as of a certain patch. This means, to me, that your spell color becoming too common is a bit confusing, because every other class and spec combo in the game has the same. Makes it odd that only warlocks get this ability to change how it looks.