Here we have 2 tables, one is in the header and another one is outside of the header. (content of the page)
As you can see, I can simply change the background color of each cell in the bottom table (via Borders & Shading > Shading properties), but if I try to do the same for another table in the header, as you can see, I only get a white background.
I don't know why this is happening, it was working some hours ago, but now, I'm stock with this.
I'm using Microsoft Word 2011 in Mac OS X 10.9.5.