Omnath's Offspring

When cast, choose red or green. During your upkeep switch colours.
Whenever a land enters the battlefield under your control:
If ({r}) Summon a 3/1 elemental with haste.
If ({g}) Summon a 1/1 elemental with "Sacrifice this creature, gain {g} mana"
Just like a Hydra, when Omnath's arm was lost 2 grew back. Alas, a new creature is born.