The purity of gold is measured in karats. Each karat represents that the metal is 1/24th gold. This means 24 gold gold is 100% gold, 18k gold is 75% gold, 14k gold is 58.3% gold, and 10k gold is 41.7% gold. The amount of gold in the alloy can alter the color of the gold. You can see examples of the gold colors in the different karat amounts here.