The only natural form of gold is yellow gold. But since gold is too soft in its pure form to make jewelry, it is normally made into an alloy by mixing it with other metals. The portion of pure gold to other metals determines the Karat rating. 24K is pure gold. 18K is 75% pure. The exact nature of the other metals used determines the color. A moderate amount of copper in the alloy creates Rose Gold. A moderate amount of palladium and nickel creates white gold.