def METHOD_NAME(self, app):
    req = WMS111MapRequest(
        url="/service?",
        param=dict(
            service="WMS",
            version="1.1.1",
            bbox="-180,0,0,80",
            width="200",
            height="200",
            layers="wms_cache_transparent",
            srs="EPSG:4326",
            format="image/png",
            styles="",
            request="GetMap",
            bgcolor="0xff00a0",
        ),
    )
    resp = app.get(req, extra_environ={"mapproxy.decorate_img": to_greyscale})
    data = BytesIO(resp.body)
    assert is_png(data)
    img = Image.open(data)
    assert img.mode == "RGB"
    assert sorted(img.getcolors())[-1][1] == (94, 94, 94)