In a review of a biography of Lane, Penguin Special: The Life and Times of Allen Lane by Jeremy Lewis (2005), that Jstor made available, one reader was quoted that the Penguin edition of the Pevsner guides are "a whole cultural education in the built environment." And regarding the Penguin paperback covers, they were color-coded according to category. Orange bands were used on fiction. Here is a picture which may be old:
I am not sure of its origin but I appreciate this site publishing the above example of a fiction cover. The cover of the book she is reading of course.
Green (and white obviously) covers specified crime fiction, cerise was for travel and adventure, dark blue for biographies, yellow for miscellaneous, red for drama; and purple for essays and belles lettres. Grey bands signified content about world affairs.