Test negative intensities

skimage/color/tests/test_colorlabel.py

@@ -3,8 +3,9 @@ import itertools
 import numpy as np
 from numpy import testing
 from skimage.color.colorlabel import label2rgb
+from skimage._shared.utils import all_warnings
 from numpy.testing import (assert_array_almost_equal as assert_close,
-                           assert_array_equal)
+                           assert_array_equal, assert_warns)
 
 
 def test_shape_mismatch():
@@ -123,6 +124,13 @@ def test_avg():
     assert_array_equal(out_bg, expected_out_bg)
 
 
+def test_negative_intensity():
+    with all_warnings():
+        labels = np.arange(100).reshape(10, 10)
+        image = -1 * np.ones((10, 10))
+        assert_warns(UserWarning, label2rgb, labels, image)
+
+
 if __name__ == '__main__':
     testing.run_module_suite()