Excess damage has been dealt to a creature if the damage dealt to it is greater than lethal damage. Usually, this means damage greater than its toughness, although damage already marked on the creature is taken into account.

If the target creature is an illegal target by the time Pigment Storm tries to resolve, the spell won't resolve. It won't deal damage to any player.

Pigment Storm's replacement effect is a self-replacement effect. It is applied before any other replacement effects that would change the amount of damage dealt to the target creature and its controller.