In a large microwave-safe bowl, melt margarine and marshmallows in a microwave set on HIGH for 2 minutes, stirring after 1 minute. Stir until mixture is smooth. To create different coloured KELLOGG'S* RICE KRISPIES* hearts, add food colouring to melted marshmallows before adding cereal.
Add KELLOGG'S* RICE KRISPIES* cereal, stirring until well coated. Spread cereal mixture into a 33 x 23 x 5 cm baking pan coated with cooking spray or butter. Allow cereal mixture to cool slightly. Cut into heart shapes using a heart-shaped cookie cutter or knife.