Which language learning product packaging innovation first introduced the concept of 'graduated interval recall' through color-coded flashcard systems?

Product packaging in the language learning industry has evolved dramatically over the decades. From simple dictionaries to immersive digital experiences, how companies present their language products significantly impacts consumer perception and adoption. This trivia question focuses on an innovative packaging breakthrough that changed how language learning products were marketed and consumed worldwide.

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Rosetta Stone's yellow box packaging with level indicators
Pimsleur's Progressive Recall System (PRS) package
Duolingo's original offline vocabulary card sets
Berlitz's Language Memory System boxes