Improve error message for empty string color

Author: oscargus

lib/matplotlib/colors.py

@@ -362,9 +362,7 @@ def to_rgba_array(c, alpha=None):
         pass
 
     if isinstance(c, str):
-        raise ValueError("Using a string of single character colors as "
-                         "a color sequence is not supported. The colors can "
-                         "be passed as an explicit list instead.")
+       raise ValueError(f"{c!r} is not a valid color value.")
 
     if len(c) == 0:
         return np.zeros((0, 4), float)

lib/matplotlib/tests/test_colors.py

@@ -1253,8 +1253,7 @@ def test_to_rgba_array_single_str():
 
     # single char color sequence is invalid
     with pytest.raises(ValueError,
-                       match="Using a string of single character colors as "
-                             "a color sequence is not supported."):
+                       match="'rgb' is not a valid color value."):
         array = mcolors.to_rgba_array("rgb")