The adverbial clause in the sentence is 'until she learned that food coloring is harmless', which provides information about the circumstances under which Janie started to like green Christmas cookies.
The adverbial clause in the sentence "Janie never liked green Christmas cookies until she learned that food coloring is harmless" is "until she learned that food coloring is harmless". An adverbial clause is a group of words that functions as an adverb and modifies a verb, an adjective, or another adverb.
This clause provides information about the time under which the action of the main clause took place. It contains a subject ('she') and a verb ('learned') and begins with a subordinating conjunction ('until'), which indicates that it cannot stand alone as a sentence.