As pure gold, which has 24 carats, it’s too soft for jewelry making, it’s mixed with other metals to make it stronger.
18k gold has a percentage of 75% of pure gold. This gold is called 750/1000 Gold.
Instead, 9k gold has 37.5% of pure gold. This gold is called 375/1000 Gold.
Alloyed with other metals, gold can change colour.
In-kind gold has a bright yellow colour, tending to orange and alloyed with silver, for example, it becomes white gold or pale yellow gold.
With copper it becomes pink or red.