There is, as yet, no legal or industry-accepted definition of what constitutes a'white' colour in golds and hence the trade description of''white gold' may not mean an alloy that is 'ice white' or 'detergent white'. For good technical and economic reasons, many commercial white golds are not a good white colour (usually a yellow-brownish tint) and are often rhodium-plated to improve appearance.
Rhodium is one of the platinum family of metals and has a high reflectivity and good metallic white colour and is hard with good wear properties. A thin electroplated coating is often applied to white gold jewellery to improve its white appearance.
This is legally allowed in many countries, including those with Hallmarking regulations. Such a coating if not subjected to undue abrasion, should have a lifetime of, typically, 3 years before it wears through to reveal the gold alloy underneath.