Line a 9x13-inch pan with parchment paper and coat with nonstick cooking spray.

Melt butter in a large saucepan over low heat. Add marshmallows and stir until completely melted. Remove from heat and mix in a few drops of green food coloring (to your desired color).

Place the cereal in a large bowl. Add marshmallow mixture and stir until the cereal is well coated.

Evenly and gently press the cereal mixture into the prepared pan with a greased spatula. Set aside to cool.

Once cooled, remove the treats (with the parchment paper) from the pan and place on top of a cutting board. Cut the cereal treats into shamrock shapes using a shamrock cookie cutter.