Photonics is generally regarded as the most promising industry to continue the technological evolution when electronic devices reach their fundamental limits on operating speeds and bandwidth. Nonetheless, more efficient control of light-matter interactions is required to harness its full potential. 1D and 2D photonic crystals have shown the ability to manipulate light-matter interactions. However, if all-optical devices are to be realized, the guiding of light should be in 3D to allow miniaturization. In this dissertation, we assessed the feasibility of exploiting the anomalous dispersion in 3D photonic crystals to influence various light-matter interactions in the visible range. This included both the development of two novel fabrication methods to integrate light-matter interactions in a 3D photonic crystal architecture and the characterization of the resulting materials. The most convenient and inexpensive way to fabricate 3D photonic crystals is by self-assembling colloidal particles into a crystalline structure. The spectral position and amplitude of the photonic band gap in such colloidal crystals can be tuned during fabrication, which is necessary for the systematic investigation of their influence on light-matter interactions.By templating these colloidal crystals with a doped epoxy, additional functionality can be introduced while maintaining the photonic crystal architecture. We developed a novel templating procedure to ensure uniformity and spatial control of the optical functionality, which is required for reliable and reproducible characterization. By employing this novel method, we successfully integrated fluorescence, upconversion, second- and third-harmonic generation and chirality into 3D photonic crystals. We demonstrated that the reduced density of optical states causes suppression of both fluorescence and upconversion when the photonic band gap coincided with the luminescence peak. When second-harmonic generation was induced, there were indications of wavevector matching due to the anomalous dispersion of the photonic band gap. A second templating method was developed to engineer a pass band into the photonic band gap by spin-coating a defect layer between two inverse opals. Since the defect layer can be doped to locally introduce optical functionality, a whole new range of research topics can be addressed. Although the architecture was well-defined and displayed pass bands on a microscopic scale, optimization is required to ensure long-range pass band uniformity. Concisely, we established a platform to investigate light-matter interactions inside 3D photonic crystals. This strategy was employed to demonstrate wavevector matching in harmonic generation and luminescence suppression. The options to vary the structural and functional properties of the 3D photonic crystals are plentiful when applying the described templating methods. This strong tunability leaves ample room for creativity. As such, it will help the systematic investigation and improvement of existing effects in these structures, while providing a research platform for the discovery and demonstration of novel effects. In terms of technological applications, this research will contribute to the development of highly efficient lasers, electro-optical modulators, organic light emitting diodes, organic photovoltaics, and optical switches.