Preheat oven to 350℉ (175℃). Grease the muffin or cupcake pan, line the wells with cupcake liners, and set aside.
In a medium bowl, combine flour, baking powder, and salt. Set aside.
Cream the butter and sugar at high speed in a stand or hand mixer until fluffy and starts looking white. It takes about 4 minutes.
Add eggs, one at a time, beating the mixture well between each addition.
Then add buttermilk and mix for another minute. Follow with the oil and mix for another minute.
Add the vanilla extract and vinegar; mix thoroughly.
Slowly add the flour mixture to the batter and mix until just combined. Scrape down the sides of the mixing bowl.
Divide the batter equally into 3 bowls. Add red food coloring to one bowl of batter and blue food coloring to the other. Mix gently until the colors are evenly distributed. Please don't overmix.
Spoon or pipe a tablespoon of the blue batter into the cupcake pan. Then, add a tablespoon of the white (uncolored) batter in each well 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 at 350℉ (175℃) for 25-30 minutes or until a toothpick comes out clean when inserted into a cupcake.
Let the cupcakes sit in the pan for 10-15 minutes.
Once the cupcakes have completely cooled, wrap them in plastic paper and place them in the refrigerator for at least an hour (preferably overnight). That makes frosting them so much easier.