Oracle Text

T: Add two mana in any combination of colors. Spend this mana only to cast Elemental spells or activate abilities of Elementals.

Card Rulings

10/1/2007 The mana can be two mana of the same color, or one mana of each of two different colors. The mana can’t be colorless.
10/1/2007 You can use this mana to pay an alternative cost (such as evoke) or additional cost incurred while casting an Elemental spell. It’s not limited to just that spell’s mana cost.
10/1/2007 The mana can’t be spent to activate activated abilities of Elemental sources that aren’t on the battlefield.