Pure gold is too soft to work with, so it is usually
alloyed with other metals such as
silver,
copper and
zinc. Using different amounts of these
metals will also give different colors.
Yellow gold: Traditional. Silver, copper and zinc.
White gold: Copper and silver with extra zinc, nickel, and/or palladium for the white color. Palladium is most likely used as only a thin plating.
Green Gold: Silver, copper, and zinc, but with extra silver to make it greenish. (Some times uses only silver)
Pink gold: Silver, copper, and zinc, with extra copper.
Gold purity is measured in carats. One carat is one twenty-fourth part, Fine gold is 24 carat (also karat, ct. or k.). Therefore only yellow gold can be 24 k. (100% gold).
http://hooverandstrong.com/products/millprod/casting.htm
Is a good site to see the different colors of gold.