In the category color configuration window, instead of having a category combobox and only one color shown at a time, it should show in a list all the categories the user created with a color button next to them. That way you could have a much better view of with color is available to be used.
Since the categories aren't editable from this screen, the list could be static. However there are not easy way to find where to edit those categories, so perhaps there should be a shortcut to the tag editor window from there?