  The fabrication of indented ('intaglio') or raised ('bas-relief')
sub-wavelength metamaterial patterns on a metal surface provides a mechanism
for changing and controlling the colour of the metal without employing any form
of chemical surface modification, thin-film coating or diffraction effects. We
show that a broad range of colours can be achieved by varying the structural
parameters of metamaterial designs to tune absorption resonances. This novel
approach to the 'structural colouring' of pure metals offers great versatility
and scalability for both aesthetic (e.g. jewellery design) and functional (e.g.
sensors, optical modulators) applications. We focus here on visible colour but
the concept can equally be applied to the engineering of metallic spectral
response in other electromagnetic domains.
