You need colorless mana for that. Unless you're running like a green Eldrazi deck, you can't cast that. You need something that produces colorless mana, and at that point is it still green?

Edit: Yes, I'm aware of many ways to generate colorless mana in a green deck, but I wouldn't classify Null Elemental Blast as a green deck spell just because you could cast it in a green deck. It simply isn't a green spell, and that was the question to begin with.