It is not very pure. Different base metals color gold in different hues. From wiki :
Green gold
Further information: Electrum
Green gold was known to Lydians as long ago as 860 BC under the name electrum, a naturally-occurring alloy of silver and gold.[3] It actually appears as greenish-yellow rather than green. Fired enamels adhere better to these alloys.
Cadmium can also be added to gold alloys but there are health concerns regarding its use.[6] The alloy of 75% gold, 23% copper, and 2% cadmium yields light-green 18-karat gold. The alloy of 75% gold, 15% silver, 6% copper, and 4% cadmium yields a dark-green alloy,