Consider what the back design would look like for a SECOND deck with the same faces. No changes to the actual artwork other than some color changes, enough to make the deck easily distinguishable from the original. Some projects set a stretch goal for a second deck, or simply start as a two-deck project, because of the popularity of "poker pairs" - two decks identical except for the back coloring. At a busy night of card games with friends, one deck can be in play while the other deck is being shuffled and prepared for the next game or next hand. This tradition of using two nearly-identical decks probably started with poker and bridge, but it's popular enough that many people do this regardless of the actual game played, especially when trying to fit in the maximum number of games in a limited period of time. The reason they're most commonly known as a poker pair is that most modern poker game sets include some different-colored poker chips and two such decks - same design, different back colors.
