Currently, we have 9 colour settings that make up a colour scheme for a space - which is fine for the default look & feel of Confluence.

However, once you start customising the root decorators, you need to be able to refer to additional colour settings in order to make any new colours customisable at the space level.

To illustrate with an example:
We want to add a new panel at the root decorator level for a LHS nav-bar (it #includes a page called "Navigation").
However, people at the space level will want the nav-bar colours to match their space's colour scheme. There currently arent enough colour settings to be able to do this nicely. (Trying to re-use to existing colour settings usually makes it ugly)

So, a way around this is to be able to define custom colour settings - which can be referred to in the stylesheet velocity.
OR, to add a fixed number of custom colours "Custom Colour 1", "Custom Colour 2", etc which can be used in the velocity.