The metals most commonly added to create yellow gold are silver, copper, and zinc. This unique mixture gives yellow gold it’s distinct color.
Yellow gold most closely resembles pure gold, but it is also typically softer than other alloys and has a tendency to scratch more easily. It can be polished periodically, but with each polish a small amount of metal is lost.
White gold is typically a result of some combination of gold with nickel, palladium, and silver. Its properties can vary dramatically depending on the metals and proportions used in the mix.
This is done deliberately for different purposes. For example, mixing gold with nickel makes it harder, which is perfect for rings and brooch pins. If you mix gold with a soft metal such as palladium it becomes good for soft gemstone settings where a pliable gold alloy is required.
The finished product, however, still tends to have a slight yellowish tint. That's why most white gold jewelry is plated with rhodium. Many people incorrectly mistake the color of rhodium with the color of white gold.
Rhodium is actually the metal that gives white gold its color. Not only that, it also makes white gold more durable by covering the softer yellow gold alloy with an extra protective layer.
While white gold looks great when it's new, its rhodium plating wears off over time. Then the lower yellowish layer of white gold becomes visible. You can have your jewelry re-plated with rhodium, but this can cost $25-$35 or more, and is an added cost that should be considered.
Rose gold is a mix of pure gold and copper. It is sometimes referred to as "pink" or "red" gold as well.
Rose gold was initially popular in Russia in the 19th century, and used to be called "Russian" gold. However, it is experiencing a resurgence in 21st century.
Although the names are often used interchangeably, the difference between red, rose, and pink gold is the copper content: the higher the copper content, the stronger the red coloration.