Yeah i dont get this at all.. I mean, we dont even have THAT many fire spells which have a flying-to-the-enemy-animation^^
incinerate is one of the most awesome looking fire spells in the game (seriously, a "wall" of fire (you know what i mean), moving like a snake to the enemy? point is: unique animation), and it NEEDS to be red (or well, yellow-ish). Green incinerate, no thanks!
then we have soulfire, kinda fits that its some kind of flaming .. lava .. bolt.. thingy. but yeah i guess green soulfire could be done, for me its perfectly fine though.
and then.. what is left?
they'd have to add a new spell to fit the "fel" theme, so i dont (and never did) think that warlocks need green fire.
oh crap, i almost forgot: IMP! yeah, would be badass if his fireballs were green, i have to admit that