Don't get caught up in the name. What if, through great concentration, lanterns could temporarily borrow power from one of the nearest lantern types? For instance, a Green Lantern could manipulate Blue or Yellow light for a short time. Think of each corp's ring as a doorway to a long hallway that is the emotional spectrum. The further you move through the spectrum the more your powers and required emotion changes. The true masters of this ability can go further into the spectrum than others. This could also lead to in between colors having new and different abilities. I haven't even given it a whole lot of thought, so some restrictions will have to apply. I also don't know if this has been suggested already in regular canon or in these forums. If so, I'd like to know!