In our country (US), purity of gold is labeled as karat (k). Purest gold is 24k, however, its softness makes it not suitable for making jewelry. That’s why other metals such as: silver, copper, zinc and nickel are alloyed in order to strengthen the gold. The metal/s added and how much, defines the color and karat of the gold.
In other words, the color of the metal added will show immediately when you check any gold piece. Gold has various colors, but let’s pretend you have a pink gold ring in your hands. Inside the ring reads 10k. Do you know what this means? First, that the gold was mixed with copper, zinc and silver to make yellow gold. The pink tone was accomplished using a large amount of copper. The 10 branding guarantees that 41.67% of the piece you have in your hands is gold, and the other 58.33% is copper, silver and zinc. Also indicating that the piece was made in the USA (other countries do not use 10K).
On the other hand, in Europe gold is marked according to its fineness. Pure gold would be 1000 fine; 18k gold is marked 75% or 750 fine. Therefore, an 18k gold piece made in Europe would be branded 750. The table below shows better all kinds of gold karat and fineness.