White gold is not a naturally white metal. It is gold alloyed to reduce its yellow tone, and the bright white finish most buyers see comes from a surface coating.
The alloy lightens the colour without removing it
Adding metals such as palladium, nickel, silver and zinc shifts gold's colour towards a pale, slightly warm grey.
The result is lighter than yellow gold but retains a faint tint, particularly at higher gold contents where less alloy is present to influence the colour.
Achieving a genuinely neutral white through alloying alone requires compositions that are expensive or difficult to work.
Rhodium supplies the visible finish
Rhodium is a platinum group metal that is naturally bright white, extremely hard and highly resistant to tarnish.
It is applied by electroplating in an exceptionally thin layer, which is enough to give the whole surface a uniform reflective white appearance.
Because so little metal is used per piece, the plating is inexpensive relative to the item even though rhodium itself is costly.
Plating wears rather than fails
The layer is measured in microns, so ordinary abrasion gradually removes it, most noticeably on ring shanks and other surfaces in constant contact.
As it thins, the warmer tone of the alloy shows through, and the piece appears to be discolouring when in fact the coating is simply wearing away.
Replating restores the finish and is a routine service, though repeated cycles slowly remove metal from the underlying piece through the preparatory polishing.
Alloy choice has practical consequences
Nickel based white gold alloys are hard and inexpensive, but nickel causes contact allergies in a significant number of people, and its release is regulated in some jurisdictions.
Palladium based alloys avoid that problem and produce a better natural colour, at a higher material cost and with more demanding working properties.
Which alloy sits beneath the plating therefore affects both how the piece behaves when the coating wears and who can comfortably wear it.
The alternative is a naturally white metal
Platinum is white throughout, so wear exposes more of the same metal rather than a different colour underneath.
It is denser, so a piece of the same design weighs more and costs more in material, and it develops a soft surface patina rather than staying mirror bright.
The choice between them is therefore a choice between a finish that must be maintained and one that changes character over time.