Preheat oven to 325°F. Generously grease a 12-cup bundt pan and set aside.
Prepare the cake mix batter as directed on the box.
Divide the batter: place 1 cup into each of 3 bowls. Tint one orange, one purple, and one black. Tint the remaining batter green.
Pour orange batter into the bottom of the bundt pan. Carefully pour purple batter on top, then black, then green. Do not stir or swirl.
Bake according to box directions for a bundt pan, or until a toothpick inserted in the center comes out clean. Cool in the pan for 5 minutes.
Invert onto a cooling rack set over a cookie sheet. Cool completely, about 30 minutes.
Divide frosting into 4 microwave-safe bowls. Tint each with orange, purple, black, and green. Microwave each bowl for a few seconds until smooth enough to drizzle.
Drizzle frosting colors one at a time over the cooled cake in a striping pattern. Sprinkle with Halloween sprinkles if desired.
Let frosting set before serving. Store loosely covered at room temperature.