Maybe that is because the essays leave some room for your own thoughts and imagination. Which you can add to what you read in the book.Similar to that, we can add some imaginary yellow to the book cover, and voilà: the blue and yellow turn to green :-)By the way, first I mis-read 'peasant', so I attributed the 'green' to the 'peasant', and thought thebook cover colour just coincidental (meanwhile I wondered: aren't Penguin books not all orange??).
I think the colour of the covers change according to the genre. But maybe I'm wrong... I haven't read so many Penguin books (neither this one).
Maybe blue was the new green? Or like the quote about life: No blue, no green.
It must be. Because I don't think the cover designer was colour blind... :P
Thank you for coming. All your comments make me extremely happy.