"Optoelectronics engineers have discovered how to make gold of any colour by carving tiny shapes onto its surface;"
|Metallic structural colour|
a Artistic impression of an intaglio metamaterial array of sub-wavelength single ring meta-molecules inscribed into a metal surface.
b The realization of this concept in gold: The words ‘NANO META’ seen under an optical microscope on the right are formed from arrays of 170 nm diameter rings (as shown in the electron microscope image, left) milled to a depth that increases in six steps from 60 to 200 nm across the sample.†
"When it comes to metal, you can have any colour as long as its silver. That's because the sea of electrons within most metals absorbs and emits light over the entire optical range. Rather than silver, metals are actually colourless.."
Ref: PDF arxiv.org: Controlling the Colour of Metals: Intaglio and Bas-Relief Metamaterials.