A shifting, rainbow-like colour effect that changes depending on the viewing angle.
Usually produced by structural colour — thin-film interference or diffraction from microscopic surface structures — rather than pigment, so the perceived hue changes with viewing and lighting angle.
Iridescence is not the same as a simple metallic paint finish, though metallic finishes can look superficially similar.
Soap bubbles, some beetle shells and certain butterfly wings show iridescence.
Iridescent finishes and structural colour behave differently under changing light than flat pigment colours — see Colour in Nature for real examples.