All gold is yellow gold. The mixture of other alloys turns yellow gold to white, rose or green gold.
Pure gold, 24 karat, is too soft to use for jewelry. It is for that reason that other alloys are mixed with it to make it stronger.
White Gold Alloys
Nickel or palladium is the alloy used when mixed with gold that turns the yellow gold to white gold. Some people find that they are allergic to the nickel. If allergic, the location under the ring or bracelet, for instance, will turn a black color or raised blisters will appear. Taking the jewelry off over a short period of time will cause the black to disappear and allow the blisters to heal. Because of allergic reactions, it is a good idea to wear platinum instead if it fits in your budget. Platinum comes out of the ground as a pure white metal and no one is allergic to it.
Rose and Pink Alloys
Copper is the alloy used to turn yellow gold to rose or pink gold. The more copper used, the more intense the rose color it becomes.
Green Gold Alloys
Silver added to yellow gold will turn to green gold.