Recently colloidal self-organization has been used to make 3D photonic crystals from a variety of materials. A typical approach is to induce sub-micron silica spheres to organize on an fcc lattice. This template, which is periodic on an optical length scale, then serves as a three-dimensional scaffolding into which another material may be infiltrated. Subsequent removal of the template by selective etching then yields a 3D photonic crystal, also called an inverted opal. These structures are particularly interesting since they have been predicted to exhibit an omnidirectional photonic band gap and enhanced optical nonlinearities. Here we show how to use colloidal self-organization to make three different materials: 1) semiconductors to explore high refractive index contrast, 2) metallo-dielectrics to explore coupling to plasmons, and 3) conjugated polymers to explore optical nonlinearities. We then discuss the optical properties of our photonic crystals. In particular, we use optical microscopy to probe a single crystalline domain in each of our samples. By measuring spatially resolved reflection and emission spectra, inhomogeneities due to averaging over inherent disorder (always present in self-assembled photonic crystals) can be eliminated. "Defect-free" spectra are obtained, from which the intrinsic photonic band structure can be extracted. Furthermore, this technique allows us to probe the specific band structure at higher frerquencies, up to the "4th-order" stop band. This capability is important since the omnidirectional photonic band gap should at the "2nd-order" stop band in inverted opals (i.e. between the 8th and 9th bands).