True White gold is an alloy. Gold, which is yellow, is mixed or blended with another metal such as nickle, in some rare cases platinum but you need an experienced goldsmith for that...the resulting mix is white gold...but it's white thru and thru and should not turn yellow-pieces that are rhodium plated are done for the shine-when the plating wears the metal undeneath shows and if it is white gold it shows white gold not yellow. If your setting is really white gold it should not be yellowing....
Now silver is often plated with rhodium to help reduce the need for polishing due to tarnishing and silver will often take on a yellowing cast or patina when tarnishing takes place before it turns that characteristic brownish-black.
Sounds like you may have a vermiel piece-gold plating over sterling silver. Unusual for it to be white gold vermeil for it's usually yellow gold that's used.
Diamonds are often set in sterling.
The only way you're gonna know for sure is to take the ring to a place that appraises jewelry and find out exactly what you have...you'll get an official report from them and if it contrdicts what Zales claims the ring is, you just might have a case here......they cannot sell an item as real white gold if it is not. Good Luck.
Edit: Hon, I've many pieces of white gold, some I guarentee are older than you-none have yellowed. Age has given them a soft satiny lustre, not that bright white shine but I think it adds to the charm of the piece.