Uncle Andrew creates four magic rings in The Magician’s Nephew.
The rings are central to the novel’s portal-fantasy plot. Uncle Andrew makes two yellow rings and two green rings using magical dust that came from Atlantis. The yellow rings transport a person to the Wood between the Worlds, an in-between place containing pools that lead to other universes. The green rings bring travelers back from that wood.
Uncle Andrew first tricks Polly Plummer into touching a yellow ring, sending her away. He then pressures Digory Kirke into taking another yellow ring so he can follow her, along with two green rings for their return journey. That arrangement makes the total four, not two: the colors describe different functions, rather than separate pairs of travelers.
The children later use the rings to visit Charn and accidentally bring Jadis, the future White Witch, into London and then into the newly created Narnia. Afterward, Digory plants the rings with an apple core; the resulting tree eventually provides wood for the wardrobe in The Lion, the Witch and the Wardrobe.