Start with the purple layer. Melt the butter in the microwave. Add the marshmallows and stir to coat with the melted butter. Microwave for 30-45 seconds. Stir until smooth.
Add the food coloring and stir until the color is uniform.
Stir in the rice krispies until coated.
Remove from the bowl and place on a silicone mat. Form a long tube (about 12 inches) for the purple layer. Coat your hands lightly with butter or olive oil if it is too sticky.
Then repeat the steps with the blue, green, yellow, orange, and pink layers. Place each layer over the purple tube. You can shape them into rectangles and then lift the rectangle onto the rainbow and use your hands to shape it.
Allow the rice krispies treats to cool and set. Then cut them into rainbow slices.
Video
Notes
Storage:
Store in an airtight container at room temperature for up to two days, in the refrigerator for up to six days, and in the freezer for up to six weeks.