"WHY IS MY WHITE GOLD RING TURNING YELLOW???"

This is a common question that plagues my store weekly. Not long ago jewelers discovered that yellow gold can be inexpensively turned white if they Rhodium plate the jewelery piece. This process also could be done over small diamonds which also help to hold a small burnished diamonds into place. The problem is that over time the rhodium plating wears off (especially on the shank). Also, if the jeweler applies any heat to the jewelery during the repair process, the rhodium needs to be reapplied. I don't have a clue what the current cost of rhodium plating is because I don't believe in it (there... I said it!). Needless to say, the inexpensive piece of white gold jewelery now cost considerably more to repair than a slightly higher price white gold piece.
Naturally, gold is not found in a white color, so the gold is mixed with different "metals" to give it's white appearance. The following is copied directly from my Mettallurgy of Karat Golds book:
White golds are created by replacing the silver in the traditional gold-copper-silver-zinc compositions used for 14 and 18K yellow gold with nickel. The nickel and zinc present in white golds reduce the red and yellow colors relected by the copper and gold. Unfortunately, this reduction in the red-yellow reflections is no where near 100% efficient and some red-yellows are still reflected. These nickel and zinc additions also do not increase the reflection of the green-blue-violet colors in white light. Remember that silver has a very high reflectivity for all the colors in the visible spectrum. The net result is that nickel white golds can have yellow tint and they are not as white as silver.
NO Jeweler has the right to say that all white gold is actually rhodium plated yellow gold. In fine white gold jewelry, the gold is not rhodium plated yellow gold, although some jewelers will rhodium plate over a true white gold to add brilliance to it's color, and that's fine. Keep in mind however, that if you have an allergy to nickel, you may want to stay away from white gold.