Sure..
When I select a color, by using the 
button, I get a dialog like this:
As you can see, I have selected some Custom colors on the right side, and added them to the lower left list by using the Add to Custom Colors button.
Now, when I close this dialog, normally the Custom colors get saved in the windows registry. I guess it would be rather tricky to do this from a web-site (security issues), but it could be saved to file, or by use of a property and an onClose method or something, I could save this Custom Colors to database.
Anyway, I would like to be able to give my customers pre-defined colors to use in their content management.
Grtz
Arjen