Which color was NOT part of the original tripartite color-coding system used for Penguin Books' brand identity in the 1930s?

Penguin Books revolutionized publishing with its distinctive branding and made literature more accessible to the masses. Their Penguin Classics line, with its instantly recognizable cover design, has become one of the most successful brand identities in literary history. This trivia question tests your knowledge of this influential publishing brand that transformed how we consume and recognize classic literature.

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Orange (for fiction)
Blue (for non-fiction)
Green (for crime fiction)
Purple (for poetry)