For the cake: Preheat the oven to 350 degrees F. Grease the bottom and sides of three 6-inch cake pans with cooking spray or butter. Line each with a 6-inch parchment round, spray with cooking spray and dust with flour.
Place the cake mix, water, oil and eggs in the bowl of a stand mixer fitted with a paddle attachment and mix on low speed for 30 seconds. Increase the speed to medium for 2 minutes. Pour the batter evenly into the prepared pans and bake until a toothpick inserted in the center comes out clean, about 30 minutes. Cool completely on a wire rack.
For the buttercream: Beat the butter in the bowl of a stand mixer fitted with a paddle attachment on high speed until fluffy, about 2 minutes. With the mixer on low speed, gradually add the confectioners' sugar and milk, 1 tablespoon at a time, alternating between the two until no lumps remain. Add the vanilla and salt and mix until incorporated.
Put 1 cup of the buttercream in 2 separate bowls and transfer the remaining buttercream to a piping bag fitted with a large round tip or a ziptop bag with the tip cut off.
Add 2 to 3 drops of the purple food coloring gel to the first bowl and stir to completely combine. Add 3 to 4 drops of the purple food coloring gel to the second bowl and stir to completely combine. Transfer the colored buttercreams to 2 piping bags fitted with a large tip or a ziptop bag with the tip cut off.
Place the first cake layer on a 6-inch cardboard round and place on a rotating cake stand. Pipe a layer of about 1/2 cup buttercream on top and use an offset spatula to spread into an even layer. Place the second cake layer on top of the buttercream and pipe another 1/2 cup buttercream on top. Spread into an even layer, then top with the third cake layer.
To apply a crumb coat, pipe about 2 cups buttercream on the top and sides of the cake and use an offset spatula or bench scraper to make smooth. Remove excess buttercream and scrape into a bowl. Transfer the cake to the refrigerator and chill for 30 minutes.
Remove the chilled cake from the refrigerator. Pipe about 2 tablespoons buttercream onto the center of a 12-inch cake drum or plate; this will help hold the cake in place while you frost. Place the cake drum on a rotating cake stand. Place the cake on top of the buttercream.
Pipe an outline of your fault line with buttercream on the top and bottom of your cake, leaving a large portion of the stripes still visible. Fill in the outline with more buttercream. Smooth with a bench scraper. Smooth the top or leave a textured edge. Chill in the fridge for another 30 minutes.
Paint the edges of the fault line with edible metallic paint.