What is the chemical difference between yellow gold and white gold?

From Wikipedia (see link to the left of this answer):

White gold is an alloy of gold and at least one white metal, such as silver or palladium. Like yellow gold, white gold is measured in Carats.

Its properties vary depending on the metals and proportions used. As a result, white gold alloys can be used for different purposes; while a nickel alloy is hard, strong and therefore good for rings and pins, white gold-palladium alloys are soft, pliable and good for white gold gemstone settings. The highest quality white gold is usually at least 17 karat, and made up of gold and palladium, and sometimes even trace levels of platinum for weight and durability, although this often requires specialised goldsmiths.

While some higher quality white gold alloys retain their shine and lustre, most will be coated with a very thin layer of rhodium. This will give the naturally more dull white gold a shine comparable to platinum or silver. Often white gold will require cleaning, or re-coating of rhodium at a later date.