Cover the surfaces of 2 baking sheets with parchment paper. Keep both sheets ready to receive the sticky cereal rings.
Warm the butter in a large saucepan over low heat, then stir in the mini marshmallows until they lose their shape. The coating should look smooth and melted without browning.
Blend the green coloring into the melted marshmallow coating. Continue stirring until the color looks uniform, with no pale streaks.
Take the saucepan off the burner, fold in the cornflakes until coated, and allow the mixture to cool slightly. Look for green-coated flakes that remain recognizable, with a mixture warm enough to shape but comfortable to handle.
Coat your hands with nonstick cooking spray, portion out 1/3 cup of cereal mixture, and shape it into a log of even thickness. The mixture should hold together without excessive squeezing, and another light spray can help whenever it starts sticking.
Curve the log into a ring, press its ends together, transfer it to a prepared sheet, and repeat with the remaining mixture. Each wreath should keep an open center and a joined seam that holds.
Press 3 cinnamon candies onto each ring and leave the wreaths undisturbed until firm, about 15 minutes, before serving. The finished rings should hold their shape when lifted from the parchment.