Which language learning product's innovative packaging design first introduced the concept of "color-coded difficulty levels" that became an industry standard in the 1980s?

The packaging of language learning products has evolved dramatically over the decades, reflecting changes in technology, teaching methodologies, and consumer preferences. This trivia question focuses on a groundbreaking packaging design that revolutionized how language learning materials were marketed and consumed by global audiences.

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Berlitz Language Pathways
Pimsleur Language Programs' Progressive Series
Rosetta Stone's Original Box Sets
Oxford Language Institute's Spectrum Collection