Preheat oven to 350℉ and grease 2 baking sheets with nonstick cooking spray. Set aside.
Set aside 5 nilla wafers for topping. Crush the rest.
10 Nilla Wafers
Prepare cake mix according to package directions. Divide batter evenly into 5 small bowls. Using the color gels, color each bowl a different color.
15.25 oz White Cake Mix, Wilton pink food coloring, Lemon yellow food coloring gel, green food coloring gel, Sky blue food coloring gel, Violet food coloring gel
Pour pink and yellow cake batter into one of the prepared pans. Bake for 6-8 minutes or until edges just begin to brown and a toothpick inserted in the center comes out clean. The cake is very thin, but should bake just fine. Cool completely.
Pour the green, blue, and violet cake batter into the second prepared pan and bake for 8-10 minutes or until the edges just begin to brown and a toothpick inserted in the center comes out clean. Cool completely.
Mix together pudding and milk and whisk well. Refrigerate for 5 minutes until set.
3.4 oz Vanilla Instant Pudding, 2 cup milk
Melt candy melts according to package directions. Dip wafer cookies halfway into the melted candy and sprinkle with one color sanding sugar. Repeat until you have 1 cookie of each color. Place on wax paper and set aside to harden.
½ cup Bright White Candy Melts, pink sanding sugar, yellow sanding sugar, green sanding sugar, blue sanding sugar, purple sanding sugar
Place 2 scoops of whipped topping into a 12" pastry bag fitted with a large star tip. Set aside.
8 oz whipped topping
To assemble trifle, use a fork to scrape the cake into crumbs keeping each color separate. Beginning with the violet cake, sprinkle it onto the bottom of the trifle bowl. Follow with half of the pudding and spread evenly on top. Sprinkle crushed wafer cookies on top. Add the blue cake and follow with half of the whipped topping. Continue with green cake and the remaining pudding. Add the rest of the crushed cookies. Spread yellow cake on top and place the remaining half of the whipped topping on top of the cake. Sprinkle with pink cake.
Use the reserved whipped topping to pipe five round dollops on top of the trifle. Press one cookie into each of the dollops.
Serve immediately or cover and refrigerate until ready to serve.
Notes
My baking pans might be bigger than the standard pans. My pans are about 12.5" x 17". If you use a smaller pan, you'll need to bake the cake longer. You can use crunchy rainbow cookies if you want to skip coloring the wafer cookies. The layers are cake, pudding, cookies, cake, whipped cream, cake, pudding, cookies, cake, whipped cream, cake, topping.