BUG : Min/max markers on box plot are not visible with 'dark_background' (#40769)

doc/source/whatsnew/v1.3.0.rst

@@ -849,6 +849,7 @@ Plotting
 - Prevent warnings when matplotlib's ``constrained_layout`` is enabled (:issue:`25261`)
 - Bug in :func:`DataFrame.plot` was showing the wrong colors in the legend if the function was called repeatedly and some calls used ``yerr`` while others didn't (partial fix of :issue:`39522`)
 - Bug in :func:`DataFrame.plot` was showing the wrong colors in the legend if the function was called repeatedly and some calls used ``secondary_y`` and others use ``legend=False`` (:issue:`40044`)
+- Bug in :meth:`BoxPlot._validate_color_args` in box plot when 'dark_background' theme was selected, caps or min/max markers for the plot was not visible (:issue:`40769`)
 
 
 Groupby/resample/rolling

pandas/plotting/_matplotlib/boxplot.py

@@ -101,7 +101,7 @@ def _validate_color_args(self):
         self._boxes_c = colors[0]
         self._whiskers_c = colors[0]
         self._medians_c = colors[2]
-        self._caps_c = "k"  # mpl default
+        self._caps_c = colors[0]
 
     def _get_colors(self, num_colors=None, color_kwds="color"):
         pass

pandas/tests/plotting/frame/test_frame_color.py

@@ -546,7 +546,13 @@ def _check_colors(bp, box_c, whiskers_c, medians_c, caps_c="k", fliers_c=None):
 
         df = DataFrame(np.random.randn(5, 5))
         bp = df.plot.box(return_type="dict")
-        _check_colors(bp, default_colors[0], default_colors[0], default_colors[2])
+        _check_colors(
+            bp,
+            default_colors[0],
+            default_colors[0],
+            default_colors[2],
+            default_colors[0],
+        )
         tm.close()
 
         dict_colors = {
@@ -569,20 +575,20 @@ def _check_colors(bp, box_c, whiskers_c, medians_c, caps_c="k", fliers_c=None):
         # partial colors
         dict_colors = {"whiskers": "c", "medians": "m"}
         bp = df.plot.box(color=dict_colors, return_type="dict")
-        _check_colors(bp, default_colors[0], "c", "m")
+        _check_colors(bp, default_colors[0], "c", "m", default_colors[0])
         tm.close()
 
         from matplotlib import cm
 
         # Test str -> colormap functionality
         bp = df.plot.box(colormap="jet", return_type="dict")
         jet_colors = [cm.jet(n) for n in np.linspace(0, 1, 3)]
-        _check_colors(bp, jet_colors[0], jet_colors[0], jet_colors[2])
+        _check_colors(bp, jet_colors[0], jet_colors[0], jet_colors[2], jet_colors[0])
         tm.close()
 
         # Test colormap functionality
         bp = df.plot.box(colormap=cm.jet, return_type="dict")
-        _check_colors(bp, jet_colors[0], jet_colors[0], jet_colors[2])
+        _check_colors(bp, jet_colors[0], jet_colors[0], jet_colors[2], jet_colors[0])
         tm.close()
 
         # string color is applied to all artists except fliers

pandas/tests/plotting/test_boxplot_method.py

@@ -195,6 +195,39 @@ def test_color_kwd(self, colors_kwd, expected):
         for k, v in expected.items():
             assert result[k][0].get_color() == v
 
+    @pytest.mark.parametrize(
+        "scheme,expected",
+        [
+            (
+                "dark_background",
+                {
+                    "boxes": "#8dd3c7",
+                    "whiskers": "#8dd3c7",
+                    "medians": "#bfbbd9",
+                    "caps": "#8dd3c7",
+                },
+            ),
+            (
+                "default",
+                {
+                    "boxes": "#1f77b4",
+                    "whiskers": "#1f77b4",
+                    "medians": "#2ca02c",
+                    "caps": "#1f77b4",
+                },
+            ),
+        ],
+    )
+    def test_colors_in_theme(self, scheme, expected):
+        # GH: 40769
+        df = DataFrame(np.random.rand(10, 2))
+        import matplotlib.pyplot as plt
+
+        plt.style.use(scheme)
+        result = df.plot.box(return_type="dict")
+        for k, v in expected.items():
+            assert result[k][0].get_color() == v
+
     @pytest.mark.parametrize(
         "dict_colors, msg",
         [({"boxes": "r", "invalid_key": "r"}, "invalid key 'invalid_key'")],