Yellow gold

Pure gold is slightly reddish yellow in color, but colored gold in various other colors can be produced.

Colored golds can be classified to three groups:

Pure 100% gold is 24 karat by definition, so all colored golds are less than this, with the common being 18K (75%), 14K (58%), and 9K (38%).

White gold is an alloy of gold and at least one white metal, usually nickel, manganese or palladium. Like yellow gold, the purity of white gold is given in karats.

White gold's properties vary depending on the metals and proportions used. As a result, white gold alloys can be used for many different purposes; while a nickel alloy is hard and strong and therefore good for rings and pins, gold-palladium alloys are soft, pliable and good for white gold gemstone settings, sometimes with other metals like copper, silver, and platinum for weight and durability, although this often requires specialized goldsmiths. The term white gold is used very loosely in the industry to describe karat gold alloys with a whitish hue. It is a common misconception that the color of the rhodium plating, which is seen on many commercial pieces, is actually the color of white gold. The term "white" covers a large spectrum of colors that borders or overlaps pale yellow, tinted brown, and even very pale rose. The jewelry industry often conceals these off-white colors by rhodium plating.

A common white gold formulation consists of 90 wt.% gold and 10 wt.% nickel. Copper can be added to increase malleability.

The strength of gold–nickel–copper alloys is caused by formation of two phases, a gold-rich Au–Cu, and a nickel-rich Ni–Cu, and the resulting hardening of the material.

The alloys used in jewelry industry are gold–palladium–silver and gold–nickel–copper–zinc. Palladium and nickel act as primary bleaching agents for gold; zinc acts as a secondary bleaching agent to attenuate the color of copper.

The nickel used in some white gold alloys can cause an allergic reaction when worn over long periods (also notably on some wrist-watch casings). This reaction, typically a minor skin rash from nickel dermatitis, occurs in about one out of eight people and because of this, many countries do not use nickel in their white gold formulations.

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Wikipedia