Until the ability that turns the Keyrune into a creature resolves, the Keyrune is colorless.

Activating the ability that turns the Keyrune into a creature while it’s already a creature will override any effects that set its power and/or toughness to another number, but effects that modify power and/or toughness without directly setting them will still apply.

If Izzet Keyrune becomes a creature in a way other than its own activated ability, its last ability will still work.