According to AR enthusiast Super Jezmerelda, AR cards (Augmented Reality cards, the special cards which come with the 3DS and can be used for 3DS games) work off of color. With his experiments, he has found it doesn’t have to be an exact or even solid match.
The key seems to be the black frame to provide a high contrast for the camera as it goes looking for the color. To see more of his experiments, check out his blog, Wild AR Cards.
What do you think of the technology used?