Why doesn't my white gold stay white?

White gold, being an alloy made up of primarily pure gold which of course has it's unique deep yellow color, is often found to be a very pale yellow color instead of true white. This lightening of pure gold's deep color is accomplished by adding metals that contribute to the alloys overall ability to reflect light of all wavelengths thus making it appear "white". The variation in color of different types of white gold alloys stem from the base metals chosen to do the alloying and the percentage of each used in the composition. The various alloys are designed to provide good working qualities during manufacturing as well as the desired color and can all vary somewhat as to the degree of 'whiteness'.
As a result of this variation and the apparent light 'yellowness' to some alloys, white gold jewelry is commonly given a final electroplated finish with rhodium. Rhodium is a precious white metal considered to be a part of the 'platinum group metals' along with osmium, iridium, ruthenium, and palladium. Some of these metals are candidates to be used as alloy metals for white gold along with nickel and silver.
It is the 'rhodium plated' finish that wears away sooner or later on white gold jewelry and allows the color of the actual gold alloy to show through and lets the jewelry take on a yellowish hue. Combined with the scuffing and scratching of normal wear a white gold piece can start to look very dingy. A good polishing to remove scuffing and a fresh rhodium plated finish will give the article a like-new appearance. This plating can last quite some time but ultimately depends on the amount of wear and tear the piece receives. Since most jewelers will charge for this refinishing service due to the high price of rhodium, a couple of times a year should be limit for this type maintenance as excessive polishing will wear out the jewelry prematurely.
There is one more scenario for the 'white gold' that turns colors and that is that it is in fact a yellow gold alloy article that has just been plated white to give it the appearance of white gold. Since it is not white to start with, this type of jewelry will discolor quite rapidly. If the color change was the owners choice they'll have to deal with the constant upkeep of the appearance. If the item was misrepresented as white gold by a seller they buyer may have recourse for a return or replacement of the jewelry.