by randomgerbil
Nope you're thinking it's more expensive than it is.So first you have to pay 1 Red and 1 Black to cast the spell. Then you choose one of green or white and one of red or blue and these choices decide what sort of attack you get, lets say you do want ranged fire attack then yes you choose red and white.
Now for each pair of 1 Red and 1 White crystal you convert into mana, note you still then have this mana to spend on other cards though it is gone at the end of the turn, you get 6 Ranged Fire Attack. Thus if you were to convert 2 red and 2 white crystals that would be 2 pairs converted and so you would get 12 Ranged Fire Attack.