Ok I got that part fixed with:

#contentMainWrapper
{background-color: #FFFFFF;
}

And I made sure the side column stayed the green color by using:

#navColumnOne, #navColumnTwo {
background-color: #B1B6A2;
}

But what I'd like to know now is how to get the main column to continue up with the white, so that the "Home :: All Products" navigation is also in white, without spilling over to the left column?