BUG: fix matplotlib warning on CN color

pandas/plotting/_matplotlib/style.py

@@ -56,29 +56,9 @@ def random_color(column):
         else:
             raise ValueError("color_type must be either 'default' or 'random'")
 
-    if isinstance(colors, str):
-        conv = matplotlib.colors.ColorConverter()
-
-        def _maybe_valid_colors(colors):
-            try:
-                [conv.to_rgba(c) for c in colors]
-                return True
-            except ValueError:
-                return False
-
-        # check whether the string can be convertible to single color
-        maybe_single_color = _maybe_valid_colors([colors])
-        # check whether each character can be convertible to colors
-        maybe_color_cycle = _maybe_valid_colors(list(colors))
-        if maybe_single_color and maybe_color_cycle and len(colors) > 1:
-            hex_color = [c["color"] for c in list(plt.rcParams["axes.prop_cycle"])]
-            colors = [hex_color[int(colors[1])]]
-        elif maybe_single_color:
-            colors = [colors]
-        else:
-            # ``colors`` is regarded as color cycle.
-            # mpl will raise error any of them is invalid
-            pass
+    if isinstance(colors, str) and _is_single_color(colors):
+        # GH #36972
+        colors = [colors]
 
     # Append more colors by cycling if there is not enough color.
     # Extra colors will be ignored by matplotlib if there are more colors
@@ -94,3 +74,33 @@ def _maybe_valid_colors(colors):
         colors += colors[:mod]
 
     return colors
+
+
+def _is_single_color(color: str) -> bool:
+    """Check if ``color`` is a single color.
+
+    Examples of single colors:
+        - 'r'
+        - 'g'
+        - 'red'
+        - 'green'
+        - 'C3'
+
+    Parameters
+    ----------
+    color : string
+        Color string.
+
+    Returns
+    -------
+    bool
+        True if ``color`` looks like a valid color.
+        False otherwise.
+    """
+    conv = matplotlib.colors.ColorConverter()
+    try:
+        conv.to_rgba(color)
+    except ValueError:
+        return False
+    else:
+        return True

pandas/tests/plotting/test_frame.py

@@ -170,10 +170,21 @@ def test_integer_array_plot(self):
 
     def test_mpl2_color_cycle_str(self):
         # GH 15516
-        colors = ["C" + str(x) for x in range(10)]
         df = DataFrame(randn(10, 3), columns=["a", "b", "c"])
-        for c in colors:
-            _check_plot_works(df.plot, color=c)
+        colors = ["C0", "C1", "C2", "C3", "C4", "C5", "C6", "C7", "C8", "C9"]
+        with warnings.catch_warnings(record=True) as w:
+            warnings.simplefilter("always", "MatplotlibDeprecationWarning")
+
+            for color in colors:
+                _check_plot_works(df.plot, color=color)
+
+            # if warning is raised, check that it is the exact problematic one
+            # GH 36972
+            if w:
+                match = "Support for uppercase single-letter colors is deprecated"
+                warning_message = str(w[0].message)
+                msg = "MatplotlibDeprecationWarning related to CN colors was raised"
+                assert match not in warning_message, msg
 
     def test_color_single_series_list(self):
         # GH 3486