Preheat oven to 400°F.
In a large mixing bowl, use whisk to beat sugar into butter until creamy and fluffy. Mixture should be light in color.
Slowly stir in eggs and flour, making sure each egg is thoroughly stirred in.
Mix in vanilla.
Stir in milk 1 tablespoon at a time, stirring thoroughly after each addition.
Divide mixture evenly into 6 smaller bowls.
Add food coloring in the following manner.
Bowl # 1 8 drops red
Bowl # 2 8 drops yellow 4 red
Bowl # 3 8 drops yellow
Bowl # 4 8 drops green
Bowl # 5 8 drops blue
Bowl # 6 8 drops blue 4 drops red
Mix well until the colors are evenly distributed.
Line muffin pan with muffin cups.
Starting with purple, spoon batter evenly into each of the muffin cups.
Spreading slightly, continue layering colors.
After purple, layer blue, green, yellow, orange, and red.
Bake for approximately 20 minutes at 400°F.
Top with your favorite frosting (I use a white frosting to make the colors stand out more).