In general the addition of silver will color gold green, and the addition of copper will color it red. Here is a nice red gold watch:
Omega DeVille Co-Axial Chronograph Red Gold
The case and bezel are in 18kt red gold.
Black gold is a type of gold used in jewelry. Black colored gold can be produced by various methods:
* Electroplating, using black rhodium or ruthenium. Solutions that contain ruthenium give a slightly harder black coating than those that contain rhodium.
* Patination by applying sulfur and oxygen containing compounds.
* Plasma assisted chemical vapor deposition process involving amorphous carbon, and controlled oxidation of carat gold containing chromium or cobalt.
More recently a laser technique has been developed that renders the surface of metals deep black. A femtosecond laser pulse deforms the surface of the metal forming nanostructures. The immensely increased surface area can absorb virtually all the light that falls on it and thus rendering it deep black.
Purple and blue golds
Purple gold (also called amethyst or violet gold) is an alloy of gold and aluminium. Gold content is around 79% and can therefore be referred to as 18 carat gold. Purple gold is more brittle than other gold alloys, and a sharp blow may cause it to shatter. It is therefore usually machined and faceted to be used as a 'gem' in conventional jewellery rather than by itself. Blue gold is similarly an alloy, a mixture of gold and indium.
White gold is an alloy of gold and at least one white metal, usually nickel or palladium.