Efreet Flamepainter’s triggered ability lets you cast the spell immediately, ignoring timing restrictions. It doesn’t let you choose to cast it at a later time.

If the card has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

If you cast a spell “without paying its mana cost,” you can’t choose to cast it for any alternative costs. You can, however, pay additional costs. If the spell has any mandatory additional costs, those must be paid to cast the spell.