Simple but very pretty. Pipe large bands of colored buttercream icing. THen use an icing scraper to pull and blend the different colors around the cake. This multi-colored smear creates a beautiful, natural look that's unique every time.
I think the fondant part is cool not a big monster high fan though. Make a Monster High cake featuring Skullette and marbled pink, purple and teal fondant. It's a simple technique that fondant beginners can try.