Most colours around result from light being absorbed (electronic transitions) and scattered by dyes and/or pigments. However, there was an interesting article in The Guardian today about structural colour. Structural colour is quite common in nature; it occurs when light is scattered because of a regularity in packing or structure; the wavelength of the light that is most strongly scattered is determined by the repeat distance of this packing, which has to be comparable to the wavelength of the light.