Karat is the term used to measure the purity of gold. The higher the karat, the purer the gold! As you are aware of 24, 22 and 18 karat kinds of gold are available in the market and pieces of jewelry made from them.
In the West, the karat is expressed in fineness. 24 k gold is expressed as 1000 parts out of 1000 pure or fineness 1.000. 22 k is expressed as 22 divided by 24, multiplied by 1000 which will give you a fineness of 0.9166, 21 karat is 21 divided by 24 multiplied by 1000 which is 0.875 degree of fineness and similarly, 18 karat is 0.750 degree of fineness.
Are you aware of the various colours of gold?
24 karat gold has the natural warm colour of pure gold and its colour cannot be changed without changing the purity of gold to less than 24 k. Other colours of gold can be made by changing the composition of the alloy in the making of the jewelry.
Pink or Rose gold - Pink gold is made by adding more copper in the alloy composition of the gold
Green Gold - Gold is given a green colour by adding more of zinc and silver
White Gold - the white colour of gold is achieved by adding nickel or palladium
Surface colour can also be given to gold articles by electro-plating. However, this can only be a surface finish and will be worn off over a period of time.