Cita:
Firespout deals 3 damage to each creature without flying if

was spent to cast Firespout and 3 damage to each creature with flying if

was spent to cast it. (Do both if

{G} was spent.)
Se è stato speso

fa danni alle non volanti.
Se è stato speso

fa danni alle volanti.
Se sono stati spesi

+{G} fa danni ad entrambe.
Direi che è logico pensare che se non è stato speso nè

nè

non venga applicato alcun effetto.
Inoltre, sempre dal link su magiccards:
Cita:
* 5/1/2008: The spell cares about what mana was spent to pay its total cost, not just what mana was spent to pay the hybrid part of its cost.
* 5/1/2008: The spell checks on resolution to see if any mana of the stated colors was spent to pay its cost. If so, it doesn't matter how much mana of that color was spent.
* 5/1/2008: If the spell is copied, the copy will never have had mana of the stated color paid for it, no matter what colors were spent on the original spell.
Con rif. alla terza, nel nostro caso la spell non è copiata ma penso valga la stessa considerazione.