BUG: Allow RGB images to be passed to grey2rgb.

skimage/color/colorconv.py

@@ -525,11 +525,14 @@ def gray2rgb(image):
         If the input is not 2-dimensional.
 
     """
-    if image.ndim != 2:
-        raise ValueError('Gray-level image should be two-dimensional.')
-
-    M, N = image.shape
-    return np.dstack((image, image, image))
+    if image.ndim > 2:
+        return image
+    elif image.ndim == 2:
+        M, N = image.shape
+        return np.dstack((image, image, image))
+    else:
+        raise ValueError('Gray-level image should be two-dimensional, '
+                         'RGB or RGBA.')
 
 
 def xyz2lab(xyz):

skimage/color/tests/test_colorconv.py

@@ -196,5 +196,12 @@ def test_gray2rgb():
     assert_equal(z[..., 0], x)
     assert_equal(z[0, 1, :], [128, 128, 128])
 
+
+def test_gray2rgb_rgb():
+    x = np.random.random((5, 5, 4))
+    y = gray2rgb(x)
+    assert_equal(x, y)
+
+
 if __name__ == "__main__":
     run_module_suite()