Gold jewelry is an alloy of gold and other metals such as copper, silver, nickel and zinc. Different combinations of these other metals are what give gold its different colors such as white, rose, green, etc. In the U.S., the percentage of actual gold in the alloy is expressed in karats. Note that karats is a percentage of gold while the carat is a measure of weight associated with diamonds and other gems.
Pure gold is 24k or 24 karats but is too soft to be used in jewelry and other applications. 18k gold is 75% gold, 14k is 58.5% and 10k is 41.6%. The minimum percentage or karatage to be legally sold as real gold in the U.S. is 10k or 41.6%. In Canada and Mexico those values are 9k and 8k respectively. Imported gold jewelry may be marked differently than with the k symbol. For instance, European gold is marked as a percentage multiplied by ten, e.g. 14k would be marked 585 and 18k would be marked 750 and so on.