Colors in gold, other than yellow, result from the combination of metal alloys mixed with the yellow gold to create various hues. Depending on the alloy(s) used it is possible to create white, rose, green, blue, grey and other shades of gold.
Yellow gold is the most popular alloy of gold. Pure gold is very soft and can me difficult to work in jewelry. Alloys can be added to yellow gold to enhance the yellow hue and create a harder more workable alloy. Typical yellow gold alloys are a mixture of gold, silver, copper and sometimes zinc.
White gold was invented in the 19th century where it was alloyed with palladium. It became commercially available as of 1912 in Pforzheim, Germany and gained popularity in the mid-1920's as a low cost substitute for platinum. It is an alloy of gold with copper, zinc and nickel. In more recent times the nickel in this alloy has often been replaced by a platinum family metal due to allergic reactions. The different recipes produce different alloys which, naturally, have different characteristics in terms of ductility, malleability and hardness. These differing properties make different alloys suitable for different purposes.
Rose gold is a gold alloy with a reddish color. This color comes from high amounts of copper in the alloy. 18K Rose gold usually contains 25% copper and 75% gold (note, these are weight percentages, not volume percentages!).
Gold à quatre couleurs refers to the combination of four distinct shades of gold alloy used in a single jewelry item. The colors are inlaid and soldered together to create a harmonious design. The combination of green, red, white and blue gold was the mixture most commonly utilized for jewelry design. Although the process was discovered earlier, the technique didn't really gain in popularity until c.1750.