White gold is really white?

White gold does not exist: Gold is yellow! 
.... but no alarm!

The 18K white gold, also called gold 750‰, is composed of 750 parts of yellow gold and 250 parts of other metals "white" , normally, nickel*, silver and palladium (The gold 750‰ is made to harden the natural metal that would otherwise be too ductile and soft: the jewels would deform).

So, is it possible to transform yellow gold into white gold?
Unfortunately, no!

250 cc of white paint in a bottle with 750 cc of yellow paint cannot transform all the paint into a white color!

To give to the white gold "his final appearance", by which we know it, we must cover it with a thin layer of rhodium. 
The process is called "Rhodium" and occurs through a galvanic (electrolysis) in salt of rhodium, a metal much more precious than gold.
The layer of rhodium that will cover the jewel will be of some micron (thousandths of a millimeter).

(*) Important information:
Directive 94/27 /CE
Of July 21, 2001, the gold objects made using alloys containing nickel, must have a supply of this metal, on human skin, of less than 0.5 micrograms per square centimeter of skin, in a week.