Sam Pell wrote:
Various alloys containing mixtures of silver, lead, (tin?.. it's been a long time since I mae knives :) ), copper, AND GOLD were described. Those alloys all had one thing in common. After the alloys were made, they were applied to surfaces, smoothed, and then etched. As the etching process progressed, all the surface atoms except the gold ones were slowly etched away. That left little gold "bump" atoms sort of lumped on the surface. Kinda like tennis balls cut in half sitting on a table. Those remaining gold atoms reflected light. The etching depth determined the portion of each gold atom that was sticking out of the surface. This in turn determined the color that the whole surface reflected. And yes, by choosing the alloy components, carefully smoothing the alloy surface, and closely monitoring the etch time, you can get essentially the full visible spectrum to reflect from an alloy surface created and etched as I just described. Portions of individual gold atoms doing the reflecting.
Not only that, but relativistic effects play a role in determining the colour of gold; if one were to try and derive the colour of gold on the basis of a non-relativistic model of the gold atom, you'd determine that it look much like silver, absorbing light primarily in the ultraviolet band. As it is however, relativistic effects alter the electrons' orbits sufficiently that the energy gap between the atom's d and s shells is narrowed, and the deeper shell (the d) becomes affected by interactions that (non-relativistically) would only affect the s shell. The energy absorption band is pulled into the visible part of the spectrum, and so gold takes on the color "silver minus blue". Other relativistic effects can be observed in other atoms, generally in the lower part of the periodic table where the masses are higher; determining bulk and chemical properties of such elements from quantum chemical principles goes awry if one doesn't take into account the fact that, the bulkier the nucleus, the faster the electrons travel around it. Something lke 65%c in the case of gold. The faster they go, the heavier they get, and the closer to the nucleus their orbitals become. Morgan L. Owens "What are you looking at? Heavy electrons."