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.