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.
Structural colour is characterised by measuring reflectance at several angles of illumination and viewing, because a single reading describes only one of its appearances. The structure itself is seen by electron microscopy: layer thicknesses and spacings of a few hundred nanometres, comparable with the wavelength of light. A practical test separates it from pigment: wet the surface so that liquid fills the gaps, and a structural colour changes or vanishes until it dries, while a pigment colour stays.
A designer samples the blue of a butterfly wing or a peacock feather from a photograph and asks for it in paint.
The photograph caught one angle. An absorbing pigment of that hex value is flat and far less intense, because the original concentrates reflected light into a narrow band and direction. Interference pigments come closer, at a cost.