The self-assembly of model colloidal particulate suspensions mimics the thermodynamics of atomic crystals. Model hard-sphere colloids exhibit fluid and close-packed solid phases. The control of inter-particle interaction forces greatly expands the complexity of the self-assembled structures. It is also a means to making ordered macroporous structures, for photonic or magnetic applications. In this talk, I will show how the control of applied external fields can be used to tune the phase behaviour and to produce novel non-close-packed colloidal order. I will show too early attempts to use these ordered colloidal crystals as templates to make patterned magnetic materials.