Posted By:
brian pIf the link provided by Dave is the card in question, I would say that nothing seems wrong with the back. Old cards vary in color for different reasons--exposure to light might have faded it a little, or perhaps navy blue was running a little on the light side that printing day. I would think that you have nothing to worry about. The more one collects vintage cards the more one realizes that the world is chock full of variations from the norm, and a porcupine is just a cactus that moves.
It's late, and I'm full of folksy whimsy,
Brian