Phase separation is a fascinating physical process that is not only responsible for the internal organization of living cells but also a powerful tool for engineering the structure of functional materials. Nature has evolved the ability to control phase separation to make composite materials with an exquisite range of optical and mechanical properties. Striking examples of these are the blue colours shown by many birds. In this work, we demonstrate control over spinodal decomposition processes at the microscale using polymer matrices. We demonstrate that the length scale of the resulting composite material is tuneable and controlled by the mechanical properties of the polymer matrix. We also show that the threshold for phase separation is stabilized with increasing elasticity, and that the preferred demixing pathway is spinodal decomposition. These results open up a range of possibilities for both fundamental and applied research, including the development of polymer composites with structural colour.