Preheat oven to 350 F. Line a 12 cup muffin pan with cupcake liners. In a medium bowl, whisk together flour, salt and baking powder. In another large bowl, beat softened butter and sugar together until fluffy.
Add in eggs, milk and vanilla extract, beat until smooth. Add the flour mixture in two additions, beating after each addition until it is fully incorporated.
Divide the cake batter evenly between three bowls. Mix yellow food coloring into one of the bowls, green food coloring into another bowl, and purple food coloring into the last bowl.
Fill 1/3 of the cupcake liners with the purple cake batter, 1/3 of the green cake batter and 1/3 of the yellow cake batter. Bake for 17-20 minutes. Let cool completely.
For the cream cheese frosting, in a large bowl using either a hand or stand mixer, beat softened butter and cream cheese together until creamy. Add in powdered sugar, 1 cup at a time until all of the powdered sugar in incorporated.
Add in vanilla extract and milk as needed to thin. Frost and decorate cooled cupcakes. Store in the refrigerator until serving or for up to one week.
