Most colored materials owe their color to the absorption of light:  certain wavelengths are absorbed and others transmitted. The color arises from the remaining wavelengths that are reflected or scattered back to the observer. In nature we often see a different type of coloration, known as structural color, which comes from interference or diffraction of light and not absorption: certain wavelengths are transmitted, while others constructively interfere and are reflected.  Structural colors are common in birds and particularly in blue feathers, which consist of disordered arrays of pores that scatter light.  I will discuss our work on making synthetic systems that mimic the bird feathers. We make these materials by packing spheres into a disordered arrangement. I will present results based on experiments, scattering theory, and geometry that shed light on the physics of structural colors -- and why blue structural color is so much more common than red.