Remove the cakes from the freezer and unwrap. Place the dark purple layer on the cake board or plate, and spread a thin layer of buttercream on it with a palette knife. Keep adding cake layers (light purple, blue, green, yellow, orange, red, pink), spreading buttercream in between. Then crumb coat the entire cake in more buttercream, and then freeze the entire cake for about 10 minutes before applying the final coat of frosting, smoothing it with your cake scraper.
Then make your rainbow drip! Melt the chopped white chocolate and cocoa butter for 15 second intervals in the microwave, stirring in between, until it's fully melted and combined. Then spoon a small amount (about a teaspoon and a half) into seven small bowls. Add your pink, red, orange, yellow, green, blue, and purple oil-based gel food coloring to each bowl so you have a bit of the drip each color of the rainbow. Working quickly before the chocolate hardens, scrape each colored drip into a small piping back (7 piping bags total) or zip top baggie and, while the drip is still warm, snip the tip off of each bag and carefully apply one drip per color to the top edge of the cake. If the drip begins to cool in the bags, microwave the bags for 10-15 seconds.
Finally, pipe the remaining buttercream around the top of the cake, and add any skittles or flowers that you'd like! Keep the cake in the fridge until serving for easiest cutting (use a hot knife to cut the cake - heated with hot water or over a stove top flame).