To create the ombré colors, set out 4 bowls. Fill the first with 3/4 white candy melts and 1/4 purple. In the second, mix 1/2 purple and 1/2 white. Fill the third with 3/4 purple and 1/4 white. In the last, use all purple. (It's okay to eyeball it.)
Working in small batches for each shade, heat candy melts according to package instructions in a microwave-safe pie plate. Add about 8 to 10 mini pretzels and turn to coat both sides. Use a fork to transfer to a wire rack sprayed with nonstick spray and placed over a rimmed baking sheet to contain drips. Coat large pretzels for topper one at a time. Set aside until candy is firm, about 30 minutes. Gently release pretzels from rack with small offset spatula.
Using a serrated knife, trim domes from tops of cakes. Place one cake layer on serving platter, and top with about 3/4 cup frosting. Repeat with second and third cake layers. Use remaining frosting to coat top and sides, smoothing with an offset spatula. Leave a fairly thick coat of frosting on the sides.
Starting at the bottom, shingle pretzels around sides. Press gently to adhere to buttercream. Insert large pretzels into top of cake, staggering their heights.