Preheat your oven to 335 °F or 168 °C and prepare your round cake pans and grease. Then, put a round parchment paper at the bottom of each pan.
After that, combine your buttermilk with the oil in a mixing bowl and set it aside.
In a different bowl, combine the remaining buttermilk, egg whites, and vanilla. Then, whisk to break up the eggs. After that, set it aside.
Next, use an electric mixer attached with a paddle attachment. And combine the flour, sugar, baking powder, baking soda, and salt for ten seconds.
Then, put the softened butter into the dry ingredients. And mix at a low speed for thirty seconds until the mixture has a coarse sand texture.
After that, add the milk and oil mixture. Mix until the dry ingredients are moist. Then, set the speed of your mixer to medium. And mix for a full two minutes.
Scrape the bowl using a rubber spatula. And reduce the speed to low. Next, add the egg white mixture in three batches. Then, mix the batter for 15 seconds between each addition. After that, scrape the bowl's sides to incorporate everything.
Divide the cake batter into six bowls, about 15 ounces each bowl. Afterward, color the batter in each bowl with the color you have chosen. Remember, 1/4 tsp for pink, 1/2 tsp for yellow, and 1/4 tsp+1/2 tsp green for green. In addition to that, it is 1/2 tsp for blue and 1/4 tsp+1/2 tsp for purple.
After that, bake your cakes for 20 to 24 minutes. Or make sure to bake until the edges start to separate from the cake pan. Because of this, do not underbake your cakes.
Afterward, instantly tap the pan firmly on the countertop once to release the steam from the cake. Hence, this trick will stop the cake from shrinking.
Finally, allow the cakes to cool for 10 minutes in the pan before turning it over. Make sure to chill your cakes before trimming.