Colour produced by microscopic physical structures scattering or reflecting light, rather than by pigment.
Structural colour arises from interference, diffraction or scattering effects at length scales comparable to visible-light wavelengths, rather than from selective light absorption by a pigment molecule.
Structural colour is not the same as a metallic paint finish, though both can produce shifting, iridescent-looking effects.
A peacock feather and a butterfly wing's blue are usually structural colour, not blue pigment — see Colour in Nature for real examples.
Structural colour explains why some of the most vivid blues and iridescent effects in nature involve no blue pigment at all.