you could try Tippex... ;)
Regular 18 carat gold is actually 75% gold and 25% copper/zinc (this is done because pure gold is actually very soft). White gold is made by using silver/palladium instead of copper/zinc. So you cant really change the colour.
However, white gold is usually rhodium plated to make it look whiter, because it's natural colour is a kind of grey... your jeweller will be able to rhodium plate the yellow gold ring, but I dont know if it would come out as white as you'd like...
Hope this helps!