14k and 18k yellow gold are made by mixing in various proportion of copper and silver (in fact, you can make up to 15 different shades of the mix). White gold is an alloy of gold and at least one white metal, usually nickel, zinc or palladium (if you are allergic to nickel, be sure to ask for palladium in alloy of white gold or stick to Platinum). The purity of white and yellow gold is given in karats. For example, alloys which are mixed 14 parts gold to 10 parts alloy create 14k gold, 18 parts gold to 6 parts alloy creates 18 karat. Interestingly,gold can be mixed to become yellow, white, rose, green and purple.