Place 2 tablespoons butter and 2 cups marshmallows in a medium, microwave safe bowl. Heat for 30 to 45 seconds, then stir to mix.
Add three to four drops of red food coloring to the bowl and stir to mix.
Fold in the cereal, stirring until all the cereal is completely covered.
Line an 8x8 pan with parchment paper and press the red mixture into the bottom of the pan in an even layer.
Wash the bowl to remove any leftover food coloring, then repeat the process with the remaining colors: orange, yellow, green, blue and purple. Layer each new color on top of the layers already in the pan.
Allow the bars to cool completely, then slice into squares and serve.