Elvish Energy Conversion Field

{g}, {t}: Add {b} to your mana pool, spend this mana only to cast Elf spells, planeswalker spells, or activate the abilities of Elves.
{b}, {t}: Add {g} to your mana pool, spend this mana only to cast Elf spells, planeswalker spells, or activate the abilities of Elves.