If you want to request a WMS layer for use as an overlay layer on top of another layer, you can configure the WMS tileset model to request transparent tile images from the server. You do so by setting the transparent option:

WMSTileSetModel.createFromURL("http://sampleservices.luciad.com/wms",
    [{layer: "rivers"}], {transparent: true})
    .then(function(model) {
      //do something with the model
    });

You can use a similar approach for a WMSImageModel.