The concept of color is more complicated than it seems. There are actually two types of colors: chemical and physical. The chemical color results from an inelastic interaction of photons with the object : we see red, blue, green according to the modified energy of the radiation after interaction. Regarding the physical color, the phenomenon is quite different: no loss of energy, but interference occurs if the object has a periodicity, which allows to "turn off" particular wavelengths in favor of some other.
There are natural objects with physical colors, such as butterfly wings or bird feathers. But now the Nanophotonics Center at the University of Cambridge has used the phenomenon giving rise to the Polymer Opal : nano-spheres with a hard heart of polystyrene and a flexible shell. These photonic crystals are introduced into an ink.
There are many applications for this stroke of genius : according to the voltage applied to the printer, tiny nano-spheres are more or less separated from each other, so the color can change during printing : one ink, several colors! Moreover, if the printed object is flexible, you can change space between nano-spheres by stretching ... and that way you can change the color of the object!