This fun and fruity cake is easy to make, once you get the hang of the blending, freezing and layering. Be sure each layer is frozen completely before adding the next to get the maximum rainbow effect. (Some layers may take longer to freeze than others.) If you want more vibrant layers, a drop or two of natural food coloring can enhance the colors. You can sub in frozen fruit if you don't have fresh on hand (just thaw before pureeing), and you can swap frozen yogurt for the ice cream if you'd like.