diff --git a/runtime/colors/lists/csscolors.vim b/runtime/colors/lists/csscolors.vim
index 74955226c2..986333d779 100644
--- a/runtime/colors/lists/csscolors.vim
+++ b/runtime/colors/lists/csscolors.vim
@@ -4,6 +4,9 @@
 " Similar in spirit to rgb.txt, this plugin establishes a human-friendly name
 " for every color listed in the CSS standard:
 "
+" Note: the color names should be in lower case, because Vim will lookup the
+" a color by its lower case name.
+"
 " https://www.w3.org/TR/css-color-3/
 
 let s:keepcpo= &cpo
@@ -26,7 +29,6 @@ call extend(v:colornames, {
 			\ 'css_blue': '#0000FF',
 			\ 'css_teal': '#008080',
 			\ 'css_aqua': '#00FFFF',
-			\
 			\ 'css_aliceblue': '#f0f8ff',
 			\ 'css_antiquewhite': '#faebd7',
 			\ 'css_aquamarine': '#7fffd4',
@@ -160,6 +162,14 @@ call extend(v:colornames, {
 			\ 'css_yellowgreen': '#9acd32',
 			\ }, 'keep')
 
+" all keys should be in lower case, convert keys that are not yet
+for [key, val] in items(filter(copy(v:colornames), { key -> key  =~ '\u'}))
+  call remove(v:colornames, key)
+  if !has_key(v:colornames, tolower(key))
+    call extend(v:colornames, {tolower(key): val}, 'keep')
+  endif
+endfor
+
 let &cpo= s:keepcpo
 unlet s:keepcpo
 
diff --git a/runtime/colors/lists/default.vim b/runtime/colors/lists/default.vim
index 7d81c6f75e..073a0da11b 100644
--- a/runtime/colors/lists/default.vim
+++ b/runtime/colors/lists/default.vim
@@ -6,6 +6,8 @@
 " time the highlight command fails to recognize a gui color. You can override
 " these colors by introducing a new colors/lists/default.vim file earlier in
 " the runtimepath.
+" Note: the color names should be in lower case, because Vim will lookup the
+" a color by its lower case name.
 
 " make sure line continuation works
 let s:keepcpo = &cpo
@@ -802,6 +804,14 @@ call extend(v:colornames, {
 			\ 'teal': '#008080'
 			\ }, 'keep')
 
+" all keys should be in lower case, convert keys that are not yet
+for [key, val] in items(filter(copy(v:colornames), { key -> key  =~ '\u'}))
+  call remove(v:colornames, key)
+  if !has_key(v:colornames, tolower(key))
+    call extend(v:colornames, {tolower(key): val}, 'keep')
+  endif
+endfor
+
 let &cpo = s:keepcpo
 unlet s:keepcpo
 
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 9c12fb8504..574b9c9dd9 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -1,4 +1,4 @@
-*eval.txt*	For Vim version 9.1.  Last change: 2024 Jan 14
+*eval.txt*	For Vim version 9.1.  Last change: 2024 Feb 05
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -2005,9 +2005,14 @@ v:collate	The current locale setting for collation order of the runtime
 								*v:colornames*
 v:colornames    A dictionary that maps color names to hex color strings. These
 		color names can be used with the |highlight-guifg|,
-		|highlight-guibg|, and |highlight-guisp| parameters. Updating
-		an entry in v:colornames has no immediate effect on the syntax
-		highlighting. The highlight commands (probably in a
+		|highlight-guibg|, and |highlight-guisp| parameters.
+
+		The key values in the dictionary (the color names) should be
+		lower cased, because Vim looks up a color by its lower case
+		name.
+
+		Updating an entry in v:colornames has no immediate effect on
+		the syntax highlighting. The highlight commands (probably in a
 		colorscheme script) need to be re-evaluated in order to use
 		the updated color values. For example: >
 
diff --git a/runtime/doc/syntax.txt b/runtime/doc/syntax.txt
index c4f59aee1b..1c9f6f2b58 100644
--- a/runtime/doc/syntax.txt
+++ b/runtime/doc/syntax.txt
@@ -1,4 +1,4 @@
-*syntax.txt*	For Vim version 9.1.  Last change: 2024 Jan 24
+*syntax.txt*	For Vim version 9.1.  Last change: 2024 Feb 05
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -5265,7 +5265,8 @@ ctermul={color-nr}				*highlight-ctermul*
 	"cterm=" argument AFTER the "ctermfg=" or "ctermbg=" argument.	Or use
 	a number instead of a color name.
 
-	The case of the color names is ignored.
+	The case of the color names is ignored, however Vim will use lower
+	case color names when reading from the |v:colornames| dictionary.
 	Note that for 16 color ansi style terminals (including xterms), the
 	numbers in the NR-8 column is used.  Here '*' means 'add 8' so that
 	Blue is 12, DarkGray is 8 etc.
@@ -5384,8 +5385,8 @@ guisp={color-name}					*highlight-guisp*
 	    :highlight Comment guifg=#11f0c3 guibg=#ff00ff
 <
 	If you are authoring a color scheme and use the same hexadecimal value
-	repeatedly, you can define a name for it in |v:colornames|. For
-	example: >
+	repeatedly, you can define a (lower case) name for it in |v:colornames|.
+	For example: >
 
 	    # provide a default value for this color but allow the user to
 	    # override it.