@playmage, I'd like to suggest evolving 'rainbow materials'. As they're rainbow materials they consist all of the other colours...
Add a material evolver to the cards screen where u can put together all five elements of the required rainbow material and 'evolve' into it...
Some examples:
Blue pearl + Red pearl + Green pearl + Light pearl + Dark pearl => Rainbow pearl
Blue map + Red map + Green map + Void map + Light map => Rainbow map
Same goes for treasure bag and treasure chest.
This wont affect the rare character of the rainbow materials, I'd say spending five materials to get one is quite costly, but it will help some of the activest players to get enough rainbow materials when they're prepared to offer some materials for it...
Would love to hear some replies on this idea