Preheat oven to 350° degrees F. Liberally grease a 13x9 rectangular light metal baking pan with cooking spray; set aside.
In a large bowl, beat together the cake mix, eggs, oil, and water with a handheld electric mixer until creamy and incorporated, about 2 minutes. Evenly divide the batter among 5 or 6 smaller bowls and tint each bowl a different color of the rainbow.
Spoon the different colored batters randomly into the pan in no particular order until all of the batter has been added to the pan. Resist the urge to swirl with a knife; this will muddy the colors.
Bake for approximately 25-30 minutes or until a toothpick inserted near the center comes out clean or with moist, but not wet, crumbs. Cool completely.
In a medium bowl, whisk together the pudding mix and milk until combined. Poke holes all over the surface of the cake using the handle of a wooden spoon, then pour the pudding mix over the holes. Cover and refrigerate for at least 2 hours to set.
Just before serving, make the whipped cream frosting. In the bowl of a stand mixer, beat the heavy whipping cream and confectioners' sugar together with a whisk attachment on HIGH speed until stiff peaks form, about 5-7 minutes. Frost the cake with the whipped cream, then garnish with the rainbow sprinkles. Store any leftover cake covered in the refrigerator.