White gold is not a naturally white metal. It is yellow gold alloyed with other metals to achieve a white look. The white colour is often further enhanced by plating the piece with rhodium. This plating could wear away, revealing the yellow-grey colour underneath.
Platinum on the other hand is a pure and naturally white precious metal which never wears away. It is more precious than gold, whether yellow or white.
Platinum jewellery is 95% pure platinum compared to 18k gold / white gold jewellery which is 75% pure gold.
You can tell the difference between platinum and white gold by their weight. Platinum is a dense metal and weighs 40% more than 18k white gold. Even a simple platinum wedding band is noticeably and reassuringly heavier than the same ring made of white gold.