A single-sided poster is white on the reverse. Generally, posters that were printed before the year 2000 will be single-sided.
A Double-sided poster has the exact same image printed on the reverse. This makes them harder and more expensive to print. An example of this can be seen in the Spectre photo in the article image. The reason posters are printed double-sided is to enable the poster to be put in a cinema lightbox. Shining a light behind a double-sided poster illuminates the colors of the poster.
Without a lightbox being used a single-sided and double-sided poster will look exactly the same from the front.
