Place 2 loaves, rounded sides down, on tray. To seal in crumbs, frost one with a thin layer of purple frosting; frost the other with a thin layer of green frosting. Cut remaining loaf crosswise in half; taper the cut edges slightly to form windshields. Place one half on each cake for cab, placing about 3 inches from front edge and with tapered windshield side toward front of car. Frost windows of cabs with a thin layer of reserved vanilla frosting; frost top and edges with a thin layer of colored frosting. Refrigerate or freeze cakes 30 to 60 minutes to set frosting. Frost entire cars with same colors.