Begin by preheating your oven to 350 degrees and adding 16-18 cupcake liners to two 12 muffin pans.
In the mixer bowl, add eggs and both granulated and brown sugar and beat for 3 minutes on high speed.
Next pour in oil, hot water, and pumpkin and mix until incorporated. Scrape sides of bowl as needed.
Turn your mixer to the slowest speed and carefully add flour, baking powder, baking soda, cinnamon, nutmeg, and salt. Mix until just combined.
Using an ice cream scooper, scoop cake batter into each cupcake liner filling only 2/3s of the way.
Bake for 16-20 minutes or until a toothpick inserted into the center comes out barely clean.
Allow the cupcakes to cool in the pans for 10 minutes then remove the cupcakes from the pans and place them on a cooling rack to cool completely.
For the Buttercream:
In your stand mixer, add butter and salt at medium low speed.
Next turn the mixer down to low and slowly add powdered sugar in small portions until all is added then turn the speed to medium high to combine.
Add heavy cream and vanilla extract and mix until nice and fluffy.
Evenly separate the buttercream into three different bowls leaving one bowl plain.
Color one bowl using the sunflower color and set aside.
Color another bowl pumpkin by using both the berry and sunflower colors.
To pipe cupcakes, start by piping one wide ring on top of cupcakes using round tip with the sunflower color then follow up on top of ring with pumpkin color with round tip and finish piping one dollop of plain color with star tip on top and serve.