Preheat the oven to 350°. Spray a 12 cup Bundt pan with vegetable oil spray.
Mix cake batter according to directions. Pour 1/2 of brown cake batter into bottom of the greased pan, then pour orange on top, finish by pouring in the remaining brown batter.
Bake for 40 minutes or until toothpick inserted in center comes out clean. Cool 5 minutes.
Turn pan upside down onto cooling rack and cool cake completely, about 30 minutes.
Divide Duncan Hines vanilla frosting in two microwaveable dishes, put 1/2 cup of chocolate frosting in a third microwaveable dish. Microwave for 15 seconds or until smooth enough to drizzle, make sure to not overheat as it will become liquid. Use the food coloring to make one orange and one purple.
With a spoon, drizzle the chocolate frosting back and forth around the whole ring in a striping pattern, then continue with purple and orange frosting.