To clarify a little on "use the opposite of what your paint base coat is, " you want to pay attention to the solvents used to make the wash, and make sure that they won't attack the color coat you already have down.  In your example, over a coat of enamels, a wash of Tamiya's X/XF acrylic, which is an alcohol-based acrylic (Momo, please correct me if I've misstated that), made with isopropyl, should be fine.  A water-based wash would work, too, like using a craft store brand like Folk Art, Americana, or Apple Barrel, and water..