Yes. Pick the paint bucket on the drawing toolbar, select "more fill colors",
click on the "custom" tab, choose whatever color you want. After you use this
colour it will appear at the bottom whenever you choose the paint bucket. Of
course, this being Word which always goes 95% of the way and then falls off a
cliff, it will only appear in the document you did this in (I'm pretty sure).
You could try saving the file as a template and see if the colour stays.