diff --git a/etc/NEWS.7.md b/etc/NEWS.7.md
index b44e7ac..9d86936 100644
--- a/etc/NEWS.7.md
+++ b/etc/NEWS.7.md
@@ -325,6 +325,10 @@ major release after 7):
   `disable_permutation_matrix` | `optimize_permutation_matrix`
   `disable_range`              | `optimize_range`
 
+  For plot functions, the use of numbers to select line colors in
+  shorthand formats was an undocumented feature that is deprecated in
+  Octave 7 and will be removed from Octave 9.
+
 - Operators
 
   Operator | Replacement | Description
@@ -362,7 +366,6 @@ from Octave 8 (or whatever version is the second major release after 6):
   and a warning is now emitted if it is used, but it will continue to
   work.
 
-
 ### Removed functions, properties, and features
 
 The following functions and properties were deprecated in Octave 5
diff --git a/scripts/help/warning_ids.m b/scripts/help/warning_ids.m
index 0215e67..0b46c00 100644
--- a/scripts/help/warning_ids.m
+++ b/scripts/help/warning_ids.m
@@ -190,6 +190,11 @@
 ## scheduled for removal from Octave.
 ## By default, the @code{Octave:deprecated-keyword} warning is enabled.
 ##
+## @item Octave:deprecated-option
+## If the @code{Octave:deprecated-option} warning is enabled, a
+## warning is issued when an obsolete option or input to a function is used.
+## By default, the @code{Octave:deprecated-option} warning is enabled.
+##
 ## @item Octave:deprecated-property
 ## If the @code{Octave:deprecated-property} warning is enabled, a
 ## warning is issued when Octave encounters a graphics property that
diff --git a/scripts/plot/util/__pltopt__.m b/scripts/plot/util/__pltopt__.m
index d91b52d..84d5e78 100644
--- a/scripts/plot/util/__pltopt__.m
+++ b/scripts/plot/util/__pltopt__.m
@@ -158,6 +158,12 @@ function [options, valid] = decode_linespec (caller, opt, err_on_invalid)
     topt = opt(1);
     n = 1;
 
+    if (any (topt == "0":"6"))
+      warning ("Octave:deprecated-option", ...
+               ["%s: using numbers to select line colors is deprecated.  ", ...
+                "Use the corresponding color identifier instead."], caller);
+    endif
+
     ## LineStyles
     if (strncmp (opt, "--", 2) || strncmp (opt, "-.", 2))
       options.linestyle = opt(1:2);
@@ -181,21 +187,28 @@ function [options, valid] = decode_linespec (caller, opt, err_on_invalid)
           n = 9;
         endif
       endif
+      ## Backward compatibility.  Leave undocumented.
+      if (topt == "@")
+        warning ("Octave:deprecated-option", ...
+                 "%s: marker type '@' is deprecated.  Use '+' instead.", ...
+                 caller);
+        topt = "+";
+      endif
       options.marker = topt;
-    ## Color specs
-    elseif (topt == "k")
+    ## Numeric color specs are for backward compatibility.  Don't document.
+    elseif (topt == "k" || topt == "0")
       options.color = [0, 0, 0];
-    elseif (topt == "r")
+    elseif (topt == "r" || topt == "1")
       if (strncmp (opt, "red", 3))
         n = 3;
       endif
       options.color = [1, 0, 0];
-    elseif (topt == "g")
+    elseif (topt == "g" || topt == "2")
       if (strncmp (opt, "green", 5))
         n = 5;
       endif
       options.color = [0, 1, 0];
-    elseif (topt == "b")
+    elseif (topt == "b" || topt == "3")
       if (strncmp (opt, "black", 5))
         options.color = [0, 0, 0];
         n = 5;
@@ -210,17 +223,17 @@ function [options, valid] = decode_linespec (caller, opt, err_on_invalid)
         n = 6;
       endif
       options.color = [1, 1, 0];
-    elseif (topt == "m")
+    elseif (topt == "m" || topt == "4")
       if (strncmp (opt, "magenta", 7))
         n = 7;
       endif
       options.color = [1, 0, 1];
-    elseif (topt == "c")
+    elseif (topt == "c" || topt == "5")
       if (strncmp (opt, "cyan", 4))
         n = 4;
       endif
       options.color = [0, 1, 1];
-    elseif (topt == "w")
+    elseif (topt == "w" || topt == "6")
       if (strncmp (opt, "white", 5))
         n = 5;
       endif