This page tells you how the different colours are used in Enterprise Theme or in the default confluence setup.
There are two different kinds of colours, the confluence standard colours and the addtional Enterprise Theme colours. Both are used in a different context or different cases. This Page will help you in case you want to adjust a specific colour. You can see here where the colours are used without changing and trying out all of the 33 colours.
Here is an explanation how the standard confluence colours are used in the Documentation Theme:
In the following section examples are listed, how the colours are used in confluence. In the example pictures the described colour is displayed in red.
Documentation of the Confluence colours