Over medium-high heat, melt 1/4 cup of butter in a tall saucepan. Add 5 cups of marshmallows. Put in 2-4 drops of green food coloring. Stir until marshmallows are melted.
Remove from heat. Stir in cereal mixture.
Dump the mixture on a baking sheet. Spray a shamrock cookie cutter with cooking spray and cut out your shapes.