How about going to another jeweller and ask if it's white gold and how much it would retail for.
Without mentioning names you could also explain what has happened , if the answers you get do not correspond with the facts. They should be able to tell you. Then at least you will have some knowledge, if you wish to go back to the original jeweller.
If the ring was brand new and sold as yellow gold when it is white gold, it must come under the 'Misrepresentation in the Sale
of goods' act. I hope you have your receipt.
White gold only goes yellow with age according to this; http://www.doamore.com/gold-vs-platinum-metal-for-your-ring/