There's Red and Blue, but why not Yellow?

[Viladin]

My first thought is that a Yellow mage would use Lightning and somehow link their magic with technology. Sort of like Artificers in the Eberron campaign setting in DnD. Though Final Fantasy d20 has the Engineer class which handles that.

Otherwise... I can't really think of any unique and good sort of combat oriented mage type. Maybe yellow mages focus on some sort of non-combat magic? Like charging up magical batteries, using all the "double money", and "double exp" boosts, and maybe teleporting people outside of battle.

The "yellow" could signify that they prefer to avoid fighting and are only in it for the money.

Or... we go way off base, and base the Mages on Lantern Corps! >.>