Can you play a spell on your opponent's turn?
No, this is explicitly forbidden in rules
According to section &quot;Playing cards → Rainbow cards&quot; of the rule book (page 5):

Creatures, Buildings, and Spells with an Action cost of 0 are always Rainbow cards. They may only be played during your own turn (even after spending your 2 Actions), but require no Landscape types to play.


Link to the rule book is taken from this page