In the metallurgy an alloy is a solid mixture of a metal with one or more other elements but mostly metals.
Yellow Gold. Yellow Gold is basically the natural and pure color of gold. Also, a small amount of other materials, color variations are obtained. Generally this is 50% copper and 50% silver
White Gold. White gold has a cooler look and include alloys such as nickel, zinc, copper, tin, palladium or manganese. White gold is through this addition is always a bit more expensive than yellow gold
Red Gold. Red Gold, with a warm romantic atmosphere is usually an alloy of copper
Green Gold. Green Gold is gold with a large amount of silver or cadmium
Blue Gold. Blue Gold is gold with some iron
White Gold. Gray Gold has an addition of 5 to 20% iron.