Gold, in its most pure form, is 24 karat, meaning 24 parts out of 24 are gold or 1000 parts our of 1000 as many Europeans tend to calculate it. In its purest form, gold is quite soft, so it is often mixed with other metals in order to make it stronger. In order to make durable and long lasting gold, jewelers combine gold with other alloys that strengthen the metal and change its color. To achieve a balance between the strength of each alloy and the value of gold, jewelers have standardized several different karat gold combinations. All karats are available in white gold, yellow gold, rose gold, and green gold with the exception of 22k and 24k which are only available in yellow.
Yellow gold can be mixed with alloys to bring the purity down. This is generally done for strength reasons.
White gold is generally yellow gold mixed with nickel. The use of nickel does two things:
Makes the color white and makes the gold harder and more durable.
After alloying gold with nickel in any purity, it is still not completely white and will have a slight yellow color to it as gold comes out of the ground yellow! Often times a metal called rhodium is plated over the gold alloy to make the final product look super bright white. White gold gives off a similar shine as platinum for a fraction of the cost. However for a stronger, whiter, hypoallergenic metal, try platinum or palladium.
Rose Gold is generally yellow gold mixed with copper. The use of copper does make the color a pinkish to reddish color.
After alloying gold with copper in any purity, it is still not complete red or pink and will still have a yellow color to it.
Green gold is achieved by mixing gold and silver together. Although silver does not have a green color by itself, when mixed with gold it creates a greenish color. It is actually more of a greenish-yellow color.
Cadmium in small amounts can also be added to gold to make gold have a greenish color. Green gold does exist naturally and its use in jewelry has been around for almost 3,000 years.
There are also specialty formulas such as black gold, blue gold, red gold, and gold that has been coated with other metals!