Heat oven to 350°F (325°F for dark or nonstick pan). Spray bottom only of 15x10x1-inch pan with baking spray with flour.
Make cake batter as directed on box. Pour into pan. Bake 20 to 26 minutes (22 to 29 minutes for dark or nonstick pan) or until toothpick inserted in center comes out clean. Cool completely, about 1 hour.
Frost cake with vanilla frosting. With toothpick, lightly draw shape of roller coaster on cake (see photo). Squeeze drops of food color in several places on frosting above roller coaster outline; use knife to swirl into frosting for sky.
Tear or cut fruit snack in half lengthwise; place strips on cake to make track of roller coaster. Add pretzels for supports. Add fruit slice candies just above track for cars; place 2 brown baking bits under each car for wheels.
In small microwavable bowl, microwave chocolate frosting uncovered on Medium (50%) 15 seconds; stir. Dip top third of each candy wafer into melted frosting; top with sprinkles for hair. Place 1 wafer “face” on top of each car.
Place remaining melted frosting in small resealable food-storage plastic bag; seal bag. Cut small hole in bottom corner of bag. Add dot of frosting to each red baking bit; add to wafers for mouths. Add licorice candies on both sides of each wafer candy for arms. Store loosely covered.