Fear not, Blizzard has already thought of that, as Lead Quest Designer Dave Kosak tweeted today:
.@tsg_lab Yes, Warlocks will be able to talk to someone to toggle green fire on and off.
- Dave Kosak (@DaveKosak) February 7, 2013
So, once a warlock has completed the quest, they can pick the color of their fire for the rest of their demonic days.
Mists of Pandaria is here! The level cap has been raised to 90, many players have returned to Azeroth, and pet battles are taking the world by storm. Keep an eye out for all of the latest news, and check out our comprehensive guide to Mists of Pandaria for everything you’ll ever need to know.