What is White Gold? White Gold is basically Yellow Gold with some added minerals like Zinc to give it a Whiter look. That's why White Gold has a Yellowish cast to it, it's really Yellow Gold to begin with! Once White Metals are added to the Gold, it's then Rhodium Plated.

Rhodium Plating is a tough White Metal that is electroplated on top of the Gold so that the Yellow disappears! Almost all White Gold Jewelry is Rhodium plated to make them a brighter and more beautiful White!

What you're seeing when your rings and Jewelry are looking Yellow is the Rhodium Plating wearing off. Rubbing off through normal wear and tear.

That's the biggest down-side to White Gold, it's not a True White Metal! How fast the Rhodium wears off depends on how often you wear your Jewelry and what it comes in contact with, like chemicals, abrasions, detergents and even your own body acids!

White Gold can last a lifetime. As long as you have it replated whenever it starts to turn yellow,Take it in to your local Jeweler and have them polish and re-rhodium plate it again!
http://www.jewelry-secrets.com/Metals/Gold/White-Gold-Turns-Yellow/White-Gold-Turns-Yellow2.html

the white gold probably contains 18/24 pure gold if it is marked 18K (that seems a very high karat, especially for white gold). But all pure gold is the familiar yellow color and the way the ring is made white is either by using a white "bleaching" metal like palladium or nickel for the non-gold portion of the alloy, or by rhodium plating, or (most likely) both. Rhodium is more expensive than gold, not less, so you're not being cheated by the rhodium plating (although you should have been told about it). But I suspect your ring probably looks somewhat yellow without the rhodium plating and needs a very high quality rhodium plating job which can only be done at a properly equipped plating shop. Ask them if they are sending it out to a reputable ISO 9001 plating shop for quality plating, or whether they intend to try to plate your precious heirloom a 2nd time in a teacup in the back room as if it were a 3rd grade science project.