Preheat oven to 400ºF (200ºC). Grease and flour two 8-in. (20-cm) diameter cake pans, line the bottoms with parchment paper rounds. Pour both packages of cake mix into 1 large bowl and prepare cake batter according to package directions. Divide batter equally into 5 separate bowls. Tint each of 4 bowls with a different food colouring to create 4 batter colours: use about 5 drops each of blue, pink, green and orange (combine red and yellow food colouring to make orange). Do not add colouring to fifth bowl of batter.
Randomly spoon dollops of the different colours of batters into both prepared pans, smoothing tops. Bake (approx. 29 to 33 min.) and cool cakes according to package directions. Spread half the frosting on top of one cake, leaving sides unfrosted. Stack second cake on top; frost top with remaining frosting.
To make optional Swirled Frosting Dollops, divide the 2 tubs of frosting into 4 separate bowls. Leave one bowl of frosting untinted. Tint the other 3 bowls with different food colourings to make pink, blue and light purple frostings. Interspersing spoonfuls of the assorted colours, scoop the frostings into a piping bag fitted with large round tip (#7). Chill pastry bag about 30 min. to firm up frosting before piping dollops on cake. Chill cake until serving time to maintain shapes of dollops.
To make optional Chocolate Pretzel Nest, line inside of 4-in. (10-cm) diameter dish (about 2-in./5-cm deep) with plastic wrap. Dip pretzels in melted chocolate, stack in the bottom and up the inside walls of the bowl to form a "nest". Roll a ball of foil to place in the nest, if necessary, to create a hollow. Refrigerate 15 to 20 min. until chocolate hardens. Remove foil ball, if using. Gently loosen nest from bowl; peel off plastic wrap. Fill nest with chocolate mini eggs, or other candy, and use as a festive cake topper.
For brighter batter colours on sides of baked cakes, grease sides of cake pans by brushing on mixture equal parts vegetable oil and all-purpose flour, stirred smooth.