diff --git a/doc/api/api_changes_3.3/behaviour.rst b/doc/api/api_changes_3.3/behaviour.rst
index 7549920d1d35..70a02d81b9d4 100644
--- a/doc/api/api_changes_3.3/behaviour.rst
+++ b/doc/api/api_changes_3.3/behaviour.rst
@@ -208,3 +208,9 @@ Passing Line2D's *drawstyle* together with *linestyle* is removed
 Instead of ``plt.plot(..., linestyle="steps--")``, use ``plt.plot(...,
 linestyle="--", drawstyle="steps")``. ``ds`` is also an alias for
 ``drawstyle``.
+
+Upper case color strings
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+Support for passing single-letter colors (one of "rgbcmykw") as UPPERCASE
+characters is removed; these colors are now case-sensitive (lowercase).
diff --git a/lib/matplotlib/colors.py b/lib/matplotlib/colors.py
index d461768d737d..ad88ea6052d8 100644
--- a/lib/matplotlib/colors.py
+++ b/lib/matplotlib/colors.py
@@ -198,17 +198,11 @@ def _to_rgba_no_colorcycle(c, alpha=None):
             # This may turn c into a non-string, so we check again below.
             c = _colors_full_map[c]
         except KeyError:
-            try:
-                c = _colors_full_map[c.lower()]
-            except KeyError:
-                pass
-            else:
-                if len(orig_c) == 1:
-                    cbook.warn_deprecated(
-                        "3.1", message="Support for uppercase "
-                        "single-letter colors is deprecated since Matplotlib "
-                        "%(since)s and will be removed %(removal)s; please "
-                        "use lowercase instead.")
+            if len(orig_c) != 1:
+                try:
+                    c = _colors_full_map[c.lower()]
+                except KeyError:
+                    pass
     if isinstance(c, str):
         # hex color in #rrggbb format.
         match = re.match(r"\A#[a-fA-F0-9]{6}\Z", c)
diff --git a/lib/matplotlib/tests/test_axes.py b/lib/matplotlib/tests/test_axes.py
index 53bc10b15b3d..030615286461 100644
--- a/lib/matplotlib/tests/test_axes.py
+++ b/lib/matplotlib/tests/test_axes.py
@@ -5650,14 +5650,6 @@ def test_annotate_across_transforms():
                 arrowprops=dict(arrowstyle="->"))
 
 
-def test_deprecated_uppercase_colors():
-    # Remove after end of deprecation period.
-    fig, ax = plt.subplots()
-    with pytest.warns(MatplotlibDeprecationWarning):
-        ax.plot([1, 2], color="B")
-        fig.canvas.draw()
-
-
 @image_comparison(['secondary_xy.png'], style='mpl20')
 def test_secondary_xy():
     fig, axs = plt.subplots(1, 2, figsize=(10, 5), constrained_layout=True)