In fact I had a hard time calling these rainbow muffins. Besides the colours, it is nothing like a rainbow, but a splash of colours.

But who cares! They were tasty, and they brought a great deal of fun time for me and my son.

Ingredients (12 regular sized muffins):
1 1/4 cups all-purpose flour
1 teaspoon baking powder
1 teaspoon baking soda
1/4 teaspoon salt
1 stick unsalted butter, softened
1 cup granulated sugar
2 large eggs
1/2 cup milk
1 teaspoon vanilla extract
a few drops of each food colouring paste (depends on how many colours you prefer)

Directions:
Preheat oven to 375F. Grease muffin tin or line with muffin liners.
In a large bowl, sift flour, baking powder, baking soda and salt. Mix well.
In the bowl of an electric mixer fitted with the paddle attachment, cream butter on medium-high speed, until light and fluffy. Gradually add sugar and continue to beat until well combined and fluffy. Reduce the mixer speed to medium and slowly add eggs until just blended.
With the mixer on low, slowly add half the flour mixture; mix until just blended. Add the milk and vanilla extract, mix until just blended. Slowly add remaining flour mixture, scraping down sides of the bowl with a spatula, as necessary, until just blended.
Divide batter evenly into several small bowls, depending on how many colours you want.
Add food colouring paste to each bowl and mix thoroughly.
Working with one colour batter at a time, evenly distribute among prepared muffin liners. Layer with remaining colour batters.
Bake for 20 to 25 minutes. Transfer muffin tin to a wire rack and let cupcakes cool completely.