Prepare cupcakes according to cake recipe instructions. Line muffin pan with black baking liners; fill with batter, bake and cool.
Combine creamy peach icing color and brown to get peach color shown for brains. Combine red-red, rose and brown to get dark red color shown for intestines. Tint 1 cup of white icing peach using color combination. Tint 1 cup white icing dark red using combination. Prepare three decorating bags, one with tip 4 and peach brain color, one with tip 1A and peach brain color, plus one with tip 8 and dark red intestine color.
With tapered spatula and white icing, smooth ice each cupcake.
For brains: On half the cupcakes, using peach icing and tip 1A, pipe a 1 ¼-inch rounded, oval-shaped mound in center. Use tip 4 and peach icing to outline each half of brain, then pipe zig zags to fill in.
For intestines: On remaining cupcakes, pipe intestines using continuous elongated beads to create overlapping loops. Chill cupcakes until icing is firm. Mix ½ cup of piping gel with red-red and a touch of brown icing color to make it a dark, blood red. Apply gel to the intestines with a brush.