BUG: Allow rgb2grey to be called on grey-level images.

skimage/color/colorconv.py

@@ -511,6 +511,9 @@ def rgb2grey(rgb):
     >>> lena = imread(os.path.join(data_dir, 'lena.png'))
     >>> lena_grey = rgb2grey(lena)
     """
+    if rgb.ndim == 2:
+        return rgb
+
     return _convert(grey_from_rgb, rgb[:, :, :3])[..., 0]
 
 rgb2gray = rgb2grey

skimage/color/tests/test_colorconv.py

@@ -146,6 +146,9 @@ class TestColorconv(TestCase):
 
         assert_equal(g.shape, (1, 1))
 
+    def test_rgb2grey_on_grey(self):
+        rgb2grey(np.random.random((5, 5)))
+
 
 def test_gray2rgb():
     x = np.array([0, 0.5, 1])