Preheat the oven to 350°F and spray three 9-inch round cake pans with baking spray.
Prepare the white cake mix batter according to package instructions, then divide it into three bowls.
Color each portion with one of the neon food colorings: green, purple, and blue.
Pour each color into a prepared pan and bake for 25-30 minutes. Let cool completely.
To make ganache: heat 1/2 cup heavy whipping cream until hot, pour over chocolate chips, and stir until smooth. Let cool slightly.
Prepare frosting: beat softened butter until creamy, gradually mix in powdered sugar, vanilla, and heavy cream until fluffy.
Color the frosting in matching neon colors and fill a piping bag with all three colors side-by-side.
Stack cake layers with frosting in between, then apply a thin crumb coat and chill.
Frost the entire cake, drizzle with ganache, and decorate with sprinkles, candy eyeballs, or Halloween candies.