After polishing and cleaning, to obtain a really bright, white, platinum like finish on a piece of white gold jewellery it needs to be Rhodium plated.
Rhodium is a rare, very hard, white metal in the Platinum Group of metals.
The plating process is an electrolytic one where the white gold object is wired up as the positive electrode and the rhodium metal in solution is deposited on the surface as the negative electrons flow from the solution to the positively charged object.
A plating of around 15 microns is normally used.
This exact same system can be used for different colours of gold plating and black Ruthenium plating.
The only problem with plating is that the coating is not thick and will need to be re plated from time to time.