Adding whatsnew and issue reference in test (pandas-dev#20585)

Author: atulagrwl

doc/source/whatsnew/v0.24.0.txt

@@ -683,6 +683,7 @@ Plotting
 
 - Bug in :func:`DataFrame.plot.scatter` and :func:`DataFrame.plot.hexbin` caused x-axis label and ticklabels to disappear when colorbar was on in IPython inline backend (:issue:`10611`, :issue:`10678`, and :issue:`20455`)
 - Bug in plotting a Series with datetimes using :func:`matplotlib.axes.Axes.scatter` (:issue:`22039`)
+- Bug in colors selection in :func:`DataFrame.plot.bar` which causes different colors to be used when no color is specified (:issue:`20585`)
 
 Groupby/Resample/Rolling
 ^^^^^^^^^^^^^^^^^^^^^^^^

pandas/tests/plotting/test_series.py

@@ -270,6 +270,7 @@ def test_bar_user_colors(self):
         assert result == expected
 
     def test_bar_user_colors_limited(self):
+        # GH 20585
         s = Series([1, 2, 3, 4])
         ax = s.plot.bar(color=['red', 'blue'])
         result = [p.get_facecolor() for p in ax.patches]
@@ -293,6 +294,7 @@ def test_bar_user_colors_more_limited(self):
 
     @pytest.mark.parametrize("num_colors",range(0,15))
     def test_standard_color(self, num_colors):
+        # GH 20585
         colors = _get_standard_colors(num_colors=num_colors)
 
         assert len(colors) == num_colors