Gold minded from the earth is a metallic mineral and is always a rich yellow color when refined. Pure gold is quite a rich color, quite soft and very malleable. For use in jewelry, gold must be made stronger. Strength is added by making gold part of a recipe(alloy) of metals. Generally, the basic metals added to pure gold are silver and copper. Small amounts of other metals are added to make a gold alloy to serve a specific purpose with special working qualities. Once made into an alloy, gold becomes the karat gold with which we are familiar: 10k, 14k, and 18k.
Just as gold is alloyed with other metals to make jewelry karat gold, COLOR of the gold is also controlled. For instance, a larger copper content takes the metal to more pink or reddish color while adding silver creates more of a greenish gold.
The various shades of yellow gold are due to the specific mix of metals the metal refiners use.
White gold is no exception. Generally a metal with a bleaching effect on the rich yellow is added to the alloy. This metal is either nickel or palladium (a metal relative of platinum). In either case, the white gold is never quite white and the yellow of the pure gold cannot be completely eliminated. Nickel white gold might show a very slight yellow tinge when new or only after some wear. Palladium white gold is generally a more grey white color.
Generally, the yellow tint to white gold that shows more than expected is due to wear and exposure. Buffing generally restores the metal to a whiter (yet not pure white) condition.
SPECIAL NOTE: If the yellow tint showed "after wearing for a while" after buffing, exposure to chlorine such as in a pool (etc.) could be partially to blame. Chemicals like that will discolor white gold. Always keep white gold away from chlorine of any form! Environmental and bodily exposure can exaggerate the problem of yellow tinting.
It is important to know about the rodium plate. The rhodium plate is not a cause for alarm or an indication the ring is yellow gold with white plating. The presence of a rhodium plate is pretty standard industry practice. Because yellow is a small part of white gold, many ring makers and jewelers cover the white gold with a very fine plate of rhodium, a very hard and very white metal. The plate is durable and can last a year or less, depending on wear. Any jeweler worth its salt should be able to polish the ring and reapply a rhodium plate the sales person should have mentioned that over time white gold can show a yellow tint. Some show a tint of yellow when buffed and some do not, depending it seems more on the alloy the maker used than on the overall quality of the ring. Generally buffing will leave the ring in a whiter state, not more yellow.
A second or third party jeweler should be able to do a quick test on the metals to confirm the karat is the same as stamped. At the same time, they will likely be able to confirm that the stone is a diamond. Don't expect a test for free or expect a guess as to value. A professional jeweler will not guess a value and that is why they are often in the appraisal business, charging for this service and attesting with their name attached to value of the jewelry. Still, a metal test is simple enough and will likely be free to perhaps $10 max to determine "if gold" and what karat.
General Information on Gold Alloys.
10KW .418 gold, nickel, copper, zinc
14KW .585 gold, nickel, copper, zinc
18KW .752 gold, nickel, copper, zinc
14K Pall .585 gold, copper, silver, palladium
18K Pall .752 gold, copper, palladium
