All gold is naturally yellow. In order to make white gold, they put a different alloy in it, however it still will take on a faint yellow look to it before polishing. What jewelers do (I am a jeweler) is Rhodium plate the ring to make it bright white again. The inside of your ring will turn yellower quicker because the acids in your skin are causing the rhodium to wear faster. Whenever you notice the outside getting duller or yellower, go to where you bought the ring (if they have an in house jeweler) and they should be able to redo it no problem. Other jewelry stores can do it for you as well, but expect a charge.
Also, don't worry if you bought quality, because you most likely did. All white gold is rhodium plated, no matter where or when you bought it. Rhodium is a part of the platinum family, so it can withstand everyday wear and tear pretty well...if someone tells you to get your ring "dipped" they mean to re-rhodium it.
Hope this helps!