The purest form of gold is, of course, golden and is referred to as 24 karat gold. Pure gold is much too soft for use in jewellery and can even be dented by simply pressing your fingernail hard against it. Needless to say, daily wear, particularly for things like rings and bracelets, would see such jewellery bent and deformed quite quickly. So the gold must be made more durable by mixing it with another kind of metal or metals, creating a gold alloy.
As far as terminology goes, the 24 karats that make up pure gold translate to all twenty-four parts being gold. So an 18 karat gold ring is constructed of 18 parts pure gold and 6 parts something else, adding up to a total of 24 (75% gold, 25% other). The same formula can be applied to any karat of gold jewellery, such as a 14 karat gold pendant- made up of 14 parts gold and 10 parts other metals.
So what are these other metals? If the desired result for a particular piece of jewellery is still a golden colour, common metals mixed with gold include copper and zinc.
With white gold, the jeweller typically uses metals like silver, palladium, manganese, and nickel, with nickel for a time being the main bleaching agent due to its cheapness. However, nickel has fallen out of favour in some jewellery circles because it more commonly causes allergic reactions.
All that said, while the resulting piece in these cases will be bleached to more of a silvery colour, it doesn’t typically produce the vibrant silver hue commonly associated with white gold today. (Although, there are methods to achieve this that have been very recently developed.) But for the vast majority of white gold out there, the vibrant silver colour is created by coating the white gold alloy with a thin layer of rhodium, a metal in the platinum family.
The choice of rhodium comes from its bright white colour along with its extreme durability. Eventually, though, it will wear down, at which point it will reveal the yellow tint of the white gold beneath. Depending on the exact makeup of the white gold, this may be barely noticeable to extremely apparent. With unscrupulous jewellers, they may even simply use regular yellow gold alloys plated with rhodium in their “white gold” as a way to save a little money in production. The buyers would have no idea until the rhodium wore off, which takes a while.
Whatever the case, if you notice a yellow tint after a while, simply having the white gold cleaned then having a jeweller apply a new coating of rhodium returns the metal to its previous silvery shine, and usually is relatively inexpensive to have done. In some cases, jewellers even offer this service free if you originally purchased the item in question from them.