All metal used in the making of jewelry is alloyed, or mixed with other metals in order to achieve a strong consistency for every day use. When gold is mined it is much too soft for a piece of jewelry for every day use. To make it more suitable and worked into jewelry it is mixed with other metals such as copper, silver, nickel and palladium.  Since gold is naturally yellow in color, it is alloyed with nickel or palladium to give it a slightly whiter tone.  Alloying alone cannot be used to achieve a pure white sheen and therefore the process of rhodium plating is applied.  Rhodium is part of the platinum group of metals and when applied, gives white gold a vibrant  finish.
Over time and due to wear and tear, the rhodium finish will begin to wear away and a yellowish hue will begin to seep through. Having your white jewelry rhodium plated from time to time is considered normal maintenance, so do not be concerned when this begins to happen. Once rhodium is reapplied the jewelry piece will appear as new!