Combine milk, ice cream and Torani in a blender pitcher and blend until smooth. Separate your milkshake mixture into 6 parts. Add food coloring to each part using the colors of the rainbow. Starting with the red layer, fill your glass 1/6 of the way full. Place glass in freezer, allowing the layer to set for about 10 minutes before adding the next layer. Repeat process with all of the colors until you complete the rainbow!