Preheat oven to 350℉ (180℃). Grease the muffin or cupcake pan, line the wells with cupcake liners, and set it aside.
Sift the flour, baking powder, and salt into a medium bowl, and set it aside.
Cream the butter and sugar at high speed with a hand or stand mixer until fluffy and looking white. It took me about 4 minutes.
Add the eggs, one at a time, whipping the mixture well between each addition.
Then add the buttermilk and mix for another minute or two. Pour in the oil and mix for another minute.
Mix in the vanilla extract and vinegar thoroughly. Then add the dry ingredients and mix until well combined, scraping down the sides of the bowl as you go.
Divide the batter equally into 3 bowls. Add red food coloring to one bowl of batter and blue food coloring to another. Mix gently until the colors are evenly distributed. Be careful to avoid overmixing.
Spoon or pipe a tablespoon of the blue batter into each cupcake well. Then, add a tablespoon of the white (uncolored) batter in each one, and end with a tablespoon of the red batter. The muffin wells should be about ⅔ full, so they have room to rise. You can layer the colors or mix and match as you like.
Bake for 25-30 minutes at 350℉ (180℃) or until a toothpick inserted into the middle of the cupcakes comes out clean. Let them cool in the pan for about 15 minutes.
After they've completely cooled, wrap them in plastic and chill them in the refrigerator for at least an hour (preferably overnight) for easier frosting.