The most likely reasons for gold discoloration are metallic abrasion, low-quality plating and corrosion. Hard metals from other jewelry or cosmetics can turn the color of gold; the plating itself can discolor, especially to yellow, since many plates are made with rhodium rather than palladium, which is more resistant to discoloration. Finally, though gold itself never corrodes, the other alloys mixed with gold, particularly silver, do, and this dulling can appear as a discoloration of the gold itself.
The makeup or cosmetics you wear can cause discoloration in gold jewelry. Many cosmetics have hard metals that can cut the softer gold. You'll know you're wearing a cosmetic that has metals harder than gold because a dark smudge will appear where the jewelry is near the cosmetic application on the skin. The smudge appears because the hard metal in the cosmetic has been separated out by contact with gold, creating a dark, powdery substance that skin absorbs. To fix the problem, switch cosmetics or apply your makeup without the jewelry. Then wash the area of the skin where the jewelry goes.
Most gold rings are sold with a rhodium plated surface, according to David Vinson of Metal Arts Specialties. These plates are about .25 to .5 microns thick, sometimes thinner. In comparison, a human hair is 100 to 125 microns. Though rhodium is very hard, reflective and pretty, it's also porous. Thus, over time, particles can sift between the rhodium plating and affect the gold alloy or simple get lodged in the plate itself, causing discoloration. Re-plate the gold jewelry in rhodium again and you'll face the problem again, year after year. Instead, have a jeweler plate the ring in a layer of platinum and then in rhodium. It will cost about $100, but you won't have to do it again for five to seven years, depending on how regularly you wear the jewelry.
"Hairspray, perfume, perspiration, smog and other chemicals can also cause discoloration," according to Mrs. Gottrocks Fine Jewelry and Gifts. Though gold itself doesn't corrode, the metals it's mixed with to form an alloy do. Silver, copper and nickel are all common metals mixed with gold. When these metals oxidize, they look very dark. Warmth, perspiration and other moisture all precipitate the discoloration in these alloys. In fact, "sometimes the actual design of the jewelry can be an influencing factor. Wide shanks (the underside portion of the ring) have more surface area to contact abrasives or corrosives," according to Mrs. Gottrocks. The best remedy is the most expensive: have a non-porous plate material, such as platinum, protecting the gold. General care will help with the discoloration, however. Take off the ring when you wash and use absorbent powders--one's without hard metals--on the area of skin where you wear jewelry to diminish moisture contact with the jewelry.