diff --git a/lib/matplotlib/rcsetup.py b/lib/matplotlib/rcsetup.py
index 0eafec792b04..7265b084d524 100644
--- a/lib/matplotlib/rcsetup.py
+++ b/lib/matplotlib/rcsetup.py
@@ -312,10 +312,11 @@ def validate_color(s):
     if isinstance(s, str):
         if s.lower() == 'none':
             return 'none'
-        if len(s) == 6 or len(s) == 8:
-            stmp = '#' + s
-            if is_color_like(stmp):
-                return stmp
+        # Support "rrggbb"/"rrggbbaa" without leading '#' as these would be
+        # interpreted as comments in matplotlibrc.  Note that "rgb" -> "#rgb"
+        # is explicitly not supported, because of the ambiguity e.g. in "C10".
+        if len(s) in [6, 8] and {*s.lower()} <= {*"0123456789abcdef"}:
+            return f'#{s}'
 
     if is_color_like(s):
         return s
diff --git a/tutorials/colors/colors.py b/tutorials/colors/colors.py
index d5bcdc9b45b5..eb432c02d34d 100644
--- a/tutorials/colors/colors.py
+++ b/tutorials/colors/colors.py
@@ -14,6 +14,12 @@
 +--------------------------------------+--------------------------------------+
 | Case-insensitive hex RGB or RGBA     | - ``'#0f0f0f'``                      |
 | string.                              | - ``'#0f0f0f80'``                    |
+|                                      |                                      |
+| .. note::                            |                                      |
+|    In a matplotlibrc file, and only  |                                      |
+|    there, these strings must be      |                                      |
+|    given without the leading '#',    |                                      |
+|    e.g. as ``text.color: 0f0f0f``.   |                                      |
 +--------------------------------------+--------------------------------------+
 | Case-insensitive RGB or RGBA string  | - ``'#abc'`` as ``'#aabbcc'``        |
 | equivalent hex shorthand of          | - ``'#fb1'`` as ``'#ffbb11'``        |