Elemental Resonance

You get the mana whether you want it or not. If you don't spend it, it will disappear at the end of the current step (or phase).

If the enchanted permanent's mana cost is 2 generic mana white mana blue mana , this Aura's controller adds colorless mana colorless mana white mana blue mana . If its mana cost is X generic mana green mana , this Aura's controller adds green mana . If its mana cost is white or blue mana white or blue mana , this Aura's controller chooses whether to add white mana white mana , white mana blue mana , or blue mana blue mana . The choice may be different each time the ability resolves.

Only the first main phase in a turn is the "precombat main phase," even if additional main phases are generated by some effect.