How to Make It
Preheat oven to 350°F. Prepare cake mixes according to package directions, using water, oil, and eggs. Divide batter evenly among 4 medium bowls. Add royal blue food coloring gel to 1 bowl, and add teal, violet, and rose gels to remaining 3 bowls until desired colors are reached.
Dollop spoonfuls of different colored batters into 3 greased and floured 9-inch round cake pans. Bake in preheated oven until a wooden pick inserted in center comes out clean, 24 to 26 minutes.
Cool cakes in pans on wire racks 10 minutes. Invert cakes from pans onto racks, and cool completely, about 45 minutes.
Microwave candy melts in a small microwavable bowl until melted, about 1 minute, stirring every 30 seconds. Transfer to a piping bag fitted with a small round tip. Place a sheet of parchment paper on top of a sheet printed with 1-inch zodiac signs; tape parchment to hold in place. Trace zodiac signs on parchment with melted candy. Repeat tracing process with a second sheet of parchment and a sheet printed with a 5-inch sun. Let zodiac signs and sun stand until dry and no longer shiny, about 20 minutes. Spray sun with gold food coloring spray. Let stand until dry. Set aside.
Using a serrated knife, trim tops of cakes to create even layers. Place 1 cake layer on an 8-inch cake board. Spread top of cake with about 1 cup of frosting. Top with second cake layer, and spread top of second cake with about 1 cup of frosting. Top with third cake layer. Spread top and sides of cakes with remaining frosting. Chill cake until frosting has hardened, about 30 minutes.
Place fondant on a silicone baking mat, and knead until pliable, dusting with powdered sugar if necessary to prevent sticking. Roll fondant to 1/4-inch thickness. Place sheet of fondant on cake, gently smoothing and stretching to prevent creases. Trim and discard excess fondant.
Using a small paintbrush, splatter white liquid icing color on tops and sides of cake. Spray sides of cake with silver food coloring spray to resemble a galaxy. Dip a wet sea sponge in different food coloring gels, and dab on sides of cake. Sprinkle silver star glitter over cake. Place sun in the center of cake, and place zodiac signs around sun, securing with additional melted candy, if necessary.