def METHOD_NAME(self, app):
    req = (
        r"/service?LAYERs=mapnik&SERVICE=WMS&FORMAT=image%2Fpng"
        "&REQUEST=GetMap&HEIGHT=200&SRS=EPSG%3A4326"
        "&VERSION=1.1.1&BBOX=-175,-85,-172,-82&styles="
        "&WIDTH=200&&BGCOLOR=0x00ff00"
    )
    resp = app.get(req)
    data = BytesIO(resp.body)
    img = Image.open(data)
    colors = sorted(img.getcolors(), reverse=True)
    # wms request bg color
    assert colors[0] == (40000, (0, 255, 0))