Gold in pure form is usually too soft to be used as jewelry; hence the need to alloy with different metals to make it more durable. Karat denotes gold's purity (usually confused with carat; meaning diamond weight). 24K denotes 24/24 parts pure gold = 100%. 18K denotes 18/24 parts pure gold = 75% or 750. 14k denotes 14/24 parts pure gold = 58% or 585. So on and so forth...

And, yes, there is green gold. You get different colors of gold depending on the type of alloy you mix in with pure gold (which is a rich yellow). More copper leads to pink or rose gold, more iron produces black gold and more nickel produces white. The special alloy used in making green gold is silver. It produces a nice subtle tint and deepens over time as it develops its patina.

Shown here are hand-textured green gold maple leaf earrings with diamonds and detachable south sea pearl drops.