Instant, 2UU (4) Counter target spell. Clash with an opponent. If you win, at the beginning of your next main phase, you may add {X} to your mana pool, where X is that spell's converted mana cost. (Each clashing player reveals the top card of his or her library, then puts that card on the top or bottom. A player wins if his or her card had a higher converted mana cost.)
Illus. Franz Vohwinkel
10/1/2007: You decide whether or not to add {X} to your mana pool as the delayed triggered ability resolves at the beginning of your next main phase. You don't decide before then.
10/1/2007: You can't add just some of the mana to your mana pool. You either add all the {X} to your mana pool or none of it. All the mana you get this way is colorless.