The gemstone opal exhibits a brilliant visible iridescence due to the regular spacing of sub-micron colloids which comprise its structure. This natural motif can be replicated in the laboratory, and artificial opals can be cast as thin films of controlled thickness using colloidal assembly techniques. Numerous solids, ranging from polymers to metals, can be cast around the colloidal network and the colloids subsequently removed. The macroporous materials that result have arrays of spherical cavities interconnected by smaller windows. The diffractive properties of these inside-out structures are even stronger than the host opal, and can in some cases can possess nearly complete photonic band gaps at visible wavelengths. In addition, these sponge-like materials can serve as hosts for further growth of nanostructured solids with complex architectures. We demonstrate this using porous PMMA to construct solid, hollow and core-shell nanospheres in a variety of shapes.