In the bowl of a stand mixer fitted with the paddle attachment mix together sugar, flour, baking powder, and kosher salt until well combined.
In a separate bowl whisk together vegetable oil, eggs, vanilla, and milk until combined. With the mixer on low add in the wet ingredients and mixed until just combined.
Use a spatula to scrape the sides of the bowl and divide evenly among three bowls. Color one bowl yellow and one bowl orange.
Fill cupcake cases 3/4 full and bake at 350 degrees for 18-20 minutes or until a toothpick inserted in the center comes out clean.
Turn out onto a clean surface and allow to cool completely before frosting.
For the Frosting:
Cream butter at a medium speed until pale and fluffy.
Add in sifted powder sugar 1/2 cup at a time mixing well after each addition. Add in vanilla and milk to reach desired consistency.
Divide frosting into 3 bowls leaving slightly more white frosting than the other two. Color one bowl purple and one bowl orange.
Frost cooled cupcakes as directed for each design.