Place the butter in a microwave-safe bowl and melt it in the microwave for about 30 seconds.
Start with about 10 drops of food coloring, then add more to achieve the color that you like.
Spray your spatula or spoon with nonstick cooking spray to keep the marshmallows from sticking too much and work quickly to stir the cereal into the melted marshmallow mixture.
Add the marshmallow and cereal mixture to a pan that has been lined with parchment paper.
Refrigerate the bars for about 30 minutes. This will help keep them from falling apart when you cut them.