The gold is by far the most expensive metal in this alloy. Here are the most commonly used alloy types. As you can see, the gold is the most important component. You can see the the amounts of pure gold in each alloy. The color is determined by the proportion of the alloy metals silver and copper.
White gold has very little or no copper in it.
Standard yellow gold is almost half silver and copper. Red gold contains, as you can guess by now, mostly copper. This is a very basic explanation of gold alloys. It should help you to understand the terms 18 karat, or 583 gold, and the colors. In reality, there are great variations in the alloy metals.