Preheat oven to 350 degrees. Line a half sheet pan with parchment paper. Mark it into four strips on the parchment paper. I used wilton's 4 piece easy layers loaf pan pans instead. You could also use two loaf pans and bake two ate a time.
In a large bowl, add in the butter and sugar. Beat on medium until light and fluffy.
Add in the vanilla and each egg one at a time. Beat for one minute until incorporated.
In a separate bowl, sift in the flour, baking powder, and salt.
Add a third of the dry mixture into the wey mixture. Beat just until combined.
Add half of the buttermilk and beat until just combined.
Repeat adding another third of the flour then the rest of the milk. Finish with the last bit of flour.
Divide the batter evenly into 4 bowls. Add 2 -3 drops of each gel paste color into each bowl. Stir to combine.
Add each batter to a prepared pan. Spread the batter out evenly.
Place into the oven and let bake for 18 minutes or until done. Place a clean toothpick into the center of the cake and if it comes out with moist crumbs or clean, it's done. If there's still batter, let it bake longer.
Remove from the oven and let cool.
In a large mixing bowl, add in the cream cheese, and butter. Mix until smooth and incorporated.
Add in the powdered sugar and vanilla and mix until combined. Mix another minute or two until smooth and fluffy.
Add in the black gel paste color and mix until well mixed. Add more gel paste color if you want to deepen the color.
Trim the top of the cake so there is no dome.
Add the black layer of cake and then a thin layer of frosting. Make sure the frosting is even.
Add the purple layer of cake and then a thin layer of frosting. Repeat with the orange and green cake layers.
Place into the fridge until the frosting is firm. I let mine sit overnight in the fridge.
Roll the cake on its side so the layers are vertical.
Frost the cake with the remaining frosting.
Coat the sides of the cake with black sanding sugar. I placed a sheet pan under my cake pan as I coated the sides with sanding sugar so it would catch the extra sugar.