Turn into a Pumpkin

Adamant effects check what mana was actually spent to cast a spell. If an effect allows you to spend mana "as though it were mana" of any color or type, that allows you to spend mana you couldn't otherwise spend, but it doesn't change what mana you spent to cast the spell.

If an instant or sorcery spell has an adamant ability, you perform the spell's instructions in order. You don't perform the adamant instruction before the spell resolves or before any other effects printed above it.

If the target nonland permanent is an illegal target by the time Turn into a Pumpkin tries to resolve, the spell won't resolve. You won't draw a card, and you won't create a Food if you spent three blue mana to cast the spell.