Preheat oven to 350 F. Spray the bottoms and sides of two 8-inch round cake pans with baking spray. Line with parchment paper rounds. Set aside.
In a medium bowl, stir together the Fruity Pebbles and buttermilk. Set aside.
In a medium bowl, whisk to combine the flour, baking powder, and salt. Set aside.
In a large mixing bowl, combine the sugar and lemon, lime and orange zests. Rub the zests into the sugar with your fingertips or use a mixer fitted with the paddle attachment to work the zest into the sugar. This helps the zest release its oil.
Next, add the butter and oil (if using) and beat on medium speed with an electric mixer fitted with the paddle attachment til light and fluffy, about 3 minutes.
Mix in the eggs one at a time. Scrape down the sides of the bowl. Add the vanilla extract.
Add half the dry ingredients and mix with the mixer on low til just combined. Add half the buttermilk and Fruity Pebbles mixture. Mix in half the remaining dry ingredients followed by the rest of the buttermilk and Fruity Pebbles mixture.
Pour batter evenly into the prepared pans. Bake for 23-27 minutes, until a toothpick comes out clean when inserted into the center of each cake.
Set pans on a cooling rack and allow cakes to cool completely. Run a knife along the edges of the pans to release any spots that may be sticking to the pan. Place a cooling rack on top of each cake pan and invert. The cake should fall to the rack on its own.
When ready to assemble, spread an even layer of frosting on top of the first layer. Place the top layer bottom side up on top of the bottom layer. Frost the top of the cake. Frost sides as you like. I like to frost a thin layer of frosting on the sides then chill the cake in the fridge for 10-15 minutes. Next, add another layer of frosting on the sides then press some Fruity Pebbles onto the sides. Top with extra Fruity Pebbles and fresh fruit, if desired.