Well, if we look at the color that are left we have Yellow and Magenta/Red, and seeing how they did the previous one I'm guessing its going to be Yellow for Jealousy, Blue for Sadness, and Red for Passion. if you put them all together you'd kind of get love in the end.
Its also pretty obvious they're leaving red for the final one, because Misaki's eyes are red, and Rena loved her eyes, it was one of the first things she noticed she'd never get to see again after losing her colors, which would make it even better if she regained that color, say, in the confession scene to Misaki.