Which publisher's 1935 innovation in book packaging design led to standardized cover designs with color-coding by genre, revolutionizing paperback marketing?

In the publishing world, book covers serve as crucial marketing tools that can make or break a title's success. The art of book packaging design has evolved dramatically over centuries, combining visual aesthetics with strategic marketing to entice readers. This trivia question explores an iconic moment in literary packaging design that revolutionized an entire genre's market approach and influenced generations of book covers that followed.

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Penguin Books with their orange and white covers
Harlequin Books with their romantic illustrations
Faber and Faber with their typography-focused designs
Bantam Books with their pulp-style artwork