Penguin Books was founded in 1935 by Allen Lane with the purpose of inexpensively producing high quality, but affordable, paperbacks. One of Penguin's strategies was to implement standardised designs for book covers.
Penguin used a simple colour scheme, two solid bands of colour sandwiching a band of white, as very general way of identifying the genre of the book. Green covers were generally for crime novels, cerise (or pink to some) was travel and adventure, dark blue were biographies, red for drama, purple for essays, and yellow was for miscellaneous titles that did not fit into any of the above categories. The most common and most famous colour scheme was orange for Penguin's fiction. Those iconic and instantly recognisable novels were found on every reader's bookshelf from World War II to the Swinging Sixties.
These colour-coded books were not only inexpensive to manufacture but they were a marketing masterstroke. Everyone could spot a Penguin book from 20 yards away. Even today, decades after Penguin stopped issuing those designs, the orange paperbacks stand out from the crowd in every used bookshop.
These original designs are still popular today and generate a deep feeling of nostalgia among older readers. Pick up a Penguin from the colours of the rainbow.