WHEN YOU SEE Green...

Back-cover copy from Death in a White Tie by Ngaio Marsh, (Penguin books, of course, 1944):

...AS THE COVER-COLOUR OF A PENGUIN BOOK, IT MEANS THAT NINEPENCE WILL SECURE HOURS OF THRILL AND EXCITEMENT IN THE FORM OF AN OUTSTANDINGLY GOOD STORY OF CRIME OR MYSTERY.

And there follows a list of examples.