Line a square baking dish with parchment paper. In a large pot, melt together mini marshmallows, butter, and vanilla extract over medium heat. Stir frequently.

Once melted, remove from heat and mix in cereals to coat entirely. Pour into your square pan, and press down firmly so that the top is a level layer. Set aside.

Make your icing by beating powdered sugar, butter, and milk together until well blended.

Divide icing into three separate bowls, and drop a little gel color into each one, then mix to blend. Place each color in a piping bag, and pipe on lines all over your cereal bars, alternating colors as you go.

Take a scraper or large spatula, and and smooth over your frosting so you have a single layer.

Add sprinkles on top, and refrigerate for at least 1 hour. Cut into squares and serve.