Self-assembly of colloids is a powerful technique for the synthesis of novel materials. While top-down manufacturing methods can also produce structures patterned on the mesoscale, they are frequently limited in resolution, slow to employ or restricted to two-dimensional systems; in contrast, self-assembly theoretically offers full control over the three-dimensional microstructure of a material and can be tuned by varying initial conditions and external forces. Self-assembly is particularly attractive because of its prevalence in the natural world; most naturally occurring nanostructures with interesting properties are produced through self-assembly processes that can then be mimicked in synthetic systems. One such class of materials exhibits structural colour, where light is differentially scattered or reflected based not on absorption but on the physical arrangement of the material on the nanoscale. Structurally produced colours tend to be brighter and more vivid than pigment-based ones and do not fade over time. As such they have numerous potential applications not only as a source of colouration but also in next-generation non-backlit displays and optoelectronic systems. This thesis discusses the synthesis, functionalisation and self-assembly of colloids into novel functional materials, particularly those that mimic naturally occurring structural colour based on disordered and ordered systems. The functionalisation of colloids with DNA introduces a specific, tuneable and thermally reversible attractive potential between particles, making it an excellent system to explore self-assembly. DNA-coated polystyrene particles were used to investigate gel formation through a spinodal decomposition mechanism and how the resulting gel structure reproducibly depends upon suspension properties. Subsequently, the tendency of such gels to produce structural colour was investigated using both polystyrene and silica DNA-coated colloidal systems, in comparison with similar natural materials. Finally, soft solution-phase photonic crystals were assembled using highly charged colloids of a low refractive index. This system was found to exhibit isotropic structural colour tuneable throughout the visible range, coupled with high transparency.