The first thing to understand is that white gold is a mixture of yellow gold and white metals. Like mixing paints, adding white makes it a very pale yellow. A high polish increases the white appearance; and quality jewellery is often plated with rhodium (a natural white metal) to complete the finish.
With time, the jewellery can become scratched, and the plating can wear off – revealing the natural tinge of the original precious gold. This may be more obvious in jewellery of high purity (18ct) which contains 75% yellow gold (by weight).
Restoring the new appearance simply involves polishing and replating. At the same time, ask the jeweller to check the settings to make sure that all the stones are secure.