Preheat oven to 350ºF.
In a large mixing bowl, cream butter and sugar together.
Add vanilla and egg whites. Beat again until smooth.
Add flour, baking powder and salt.
Add milk. Beat again until batter is smooth.
Separate batter into 5 separate bowls. Add one of the five food coloring colors to each bowl. The amount you add will depend on the type of food coloring you are using. Stir well until colors are even.
Prepare 8-inch round baking pans by coating them with cooking spray and a thin layer of flour. Tap pan to remove excess flour.
Transfer each color of batter to a prepared baking pan.
Cook each colored layer for 15 minutes
or until a toothpick inserted in the center comes out clean.
When done baking, allow them to cool for 15 minutes before flipping them out of the pans on to a rack. You may need to use a knife to release the edges. Allow them to cool completely before frosting them.