By default, there is gutter space between floated divisions in FGLs. I've found no way around this except to compromise a little on design and to keep things very simple.
Thanks Nancy. I kind of figured that might be the answer... not sure why I'd even need gutter space - I wonder if I can just set it to zero. Then I can revert to my original fix and set the background-color to the gridContainer class. I'll give that a whirl. Cheers!