Colour that changes with the angle you view or light it from, as on metallic car paint, pearlescent finishes and some bird feathers.
Caused by flake orientation in metallic and effect paints (the lightness 'flop') or by thin-film interference in pearlescent and structural colour, where the wavelengths reinforced depend on the angle of incidence.
Goniochromatic colours cannot be described by one measurement; they are measured at several angles.
A silver car looks bright near the highlight and dark grey at a glancing angle.
Matching or photographing an effect finish needs the angles fixed, or two samples that match head-on disagree from the side.
Effect finishes are measured with a multi-angle spectrophotometer. The light strikes the surface at a fixed angle and detectors read the colour at several angles measured from the mirror direction, close to it and far from it. The result is a set of CIELAB values, one per angle, and a tolerance is set for each. The change in lightness from the near-mirror angle to the far angle is the flop, and it is controlled as closely as the colour because it depends on how the flakes lay down as the paint dried.
A bumper or a door is resprayed in a metallic colour with the correct paint code.
Spray pressure, distance and drying speed set the flake orientation, so the repair can match face-on and look lighter or darker at a glancing angle. Refinishers blend the new paint across the adjoining panels so that no hard edge shows the difference.
Plastic and metal parts of one product are finished in the same metallic colour by different suppliers.
Each matches the master at the angle it was checked and they disagree at the others, so the assembled product looks two-tone as it turns in the hand. A multi-angle specification, agreed by all suppliers, is the only control.