1. Gather ingredients and spray a 9x13" cake pan with non stick baking spray.
2. In large saucepan melt butter over medium-low heat.
3. Add marshmallows and stir until completely melted. Remove from heat.
4. Add Fruity Pebble rice krispie cereal.
5. Stir until the cereal mixture is well combined.
6. Using a buttered spatula or wax paper, evenly press the mixture into the baking dish. Allow to cool before removing from the pan and placing on a wire rack.
7. In a medium Pyrex mixing cup, combine powdered sugar and water. Stir until the icing becomes a smooth, pourable consistency.
8. Drop several drops of each color of food coloring on top of the icing in the cup.
9. Swirl the mixing cup slightly before carefully pouring the icing all over the top of the rice krispie bar.
10. Allow the icing to completely harden on the counter or in the fridge for about an hour.
11. Cut the rice treat into 2" bars with a sharp knife.