I don't know if that can be done. But I can offer kind of a work-around for Word. Define styles with the correct colors and apply those to the selected text in stead of a color from the drop down box. Obviously, this is kind of a chore if you want to apply 4 colors to a lot of different fonts, because you'll need multiple colored versions of each font and font size.
I want to make a permanent change in the drop down default color selection in Office. IE I have 3 or 4 custom colors that I use when designing materials for my company and I want them to show as default colors with out having to change RBG values each time.
I am on Office 2003 and mostly use Word, Publisher and PowerPoint.