For this site, I've been using Minnelli - a fixed-width version of Drupal 6's default Garland theme - along with the Color module to change the site's default colour set to Bluemarine as oppose to the standard Blue Lagoon.
Recently, I decided that I wanted to edit the page.tpl.php and node.tpl.php template files, but as I was using a core theme, I wasn't able to change these files. So, I created a copy of the Minelli theme directory, renamed it, and placed it into my sites/all/themes directory. As with the default Minnelli theme, my copied theme would need to use Garland as a base theme, to do this, I added the following code to its .info file:
base theme = garland
Click here for more information on sub-themes.

To allow the Color module to work with this theme, the 'Color' directory needs to be copied as part of the Minnelli theme. Once this was copied, I was able to visit admin/build/themes/settings/mytheme to configure its settings.
However, when I attempted to apply my new colour set, 4 error messages were being displayed telling me that files could not be copied as no file with that name existed. When I looked into it, the files that were missing were images contained within a seperate folder. I opened the color.inc file, and instead of changing the paths and values within it, I decided to find an alternative solution.

My solution was to remove the copied folder, and to create symlinks instead.
Opening a terminal window, I created symlinks to the original color and images directories within the Garland theme folder, as well as Garland's style.css file.
ln -s ../../../../themes/garland/color color ln -s ../../../../themes/garland/images images ln -s ../../../../themes/garland/style.css style.css


With these links created, my new theme could reference the original directories and files within the Garland folder, and apply my new theme configuration settings.