I bought a citrine gemstone ring and a pair of 1/4 carat diamond earrings from Helzberg Diamonds in 2000. Both the ring and the earrings have 10k stamped inside of them. They have not been left in any cleaning solution or subjected to crazy temperatures. The jewelry has turned copper color. I took it to a local jeweler and was told that they are not 100% gold. It is been some time since my purchase, but I thought they were a reputable company. Why would they sale fake gold? I do not care how long ago I made the purchase, it was a legal contract and they should honor it. I will be shipping both pieces of jewelry to their warehouse. I spend over $250 dollars on that purchase.

10k is not 100% gold... it's 41.7% pure gold mixed with alloys that make it stronger. 24k gold is the only 100% pure gold, but is incredibly soft which is why it isn't really used in jewelry.
As far as the color your earrings are turning, depends on if it's yellow or white gold. If it is white gold, your rhodium plating is simply wearing down. White gold is yellow coated with rhodium which is what makes it bright white. Most white gold jewelry needs to be plated every 6 months-1 year in order to maintain a "new" appearance.
Some other reasons your jewelry is losing color might be from how often you wear them and if you wear them in the shower. Showering with jewelry is not a good idea. It can loosen your stones and the products you use in the shower can build up and wear your jewelry down.
Take care of your jewelry by taking it off before showering and getting it cleaned every 6 months by a jeweler.

To answer your direct complaint, helzberg does not sell fake gold. They sell 10k, 14k, and 18k. The only gold that is 100% is 24k.