diff --git a/autoload/nuwiki/colors.vim b/autoload/nuwiki/colors.vim
index fda67e3..7943d1b 100644
--- a/autoload/nuwiki/colors.vim
+++ b/autoload/nuwiki/colors.vim
@@ -41,6 +41,13 @@ function! s:define(color) abort
     return
   endif
   let b:nuwiki_color_seen[a:color] = 1
+  " Only handle colours Vim can actually allocate: a #hex value or an
+  " alphabetic name. Anything else — rgb()/hsl() functions, or a literal
+  " `<span style="color:…">` that turns up in prose/examples — is skipped, so
+  " a stray match can't create a bogus conceal region or abort the refresh.
+  if a:color !~# '^#\x\{3,8}$' && a:color !~# '^[A-Za-z][A-Za-z0-9]*$'
+    return
+  endif
   let l:grp = 'nuwikiColorSpan_' . substitute(a:color, '[^A-Za-z0-9]', '_', 'g')
   let l:esc = escape(a:color, '/\.*$^~[]')
   " Clear first so re-runs / load-order never stack duplicate regions.
@@ -52,9 +59,10 @@ function! s:define(color) abort
         \ . ' start=/<span style="color:' . l:esc . '">/'
         \ . ' end=#</span>#'
         \ . ' oneline keepend concealends'
-  " guifg works for names and #hex under 'termguicolors'. ctermfg only accepts
-  " named colours, so try it for non-hex and swallow E254 for unknown names.
-  execute 'highlight default ' . l:grp . ' guifg=' . a:color
+  " guifg works for names and #hex under 'termguicolors'; ctermfg only for
+  " named colours. Both are best-effort — `silent!` swallows E254 for a name
+  " Vim doesn't know (the tags still conceal, the text just isn't recoloured).
+  silent! execute 'highlight default ' . l:grp . ' guifg=' . a:color
   if a:color !~# '^#'
     silent! execute 'highlight ' . l:grp . ' ctermfg=' . a:color
   endif
diff --git a/development/_common.sh b/development/_common.sh
index 1e297e7..71b2011 100644
--- a/development/_common.sh
+++ b/development/_common.sh
@@ -97,9 +97,9 @@ super^script^ and sub,,script,,, and inline math $e^{i\pi} + 1 = 0$.
 
 Put the cursor on a word and run `:VimwikiColorize red` (or press
 `<Leader>wc` and type a colour); or visually select a phrase and press
-`<Leader>wc`. Each wraps the target in `<span style="color:…">…</span>` —
-the tags are concealed, so you see just the word painted in that colour
-(move the cursor onto it to reveal the raw markup).
+`<Leader>wc`. Each wraps the target in an inline colour-span tag whose
+markup is concealed, so you see just the word painted in that colour (move
+the cursor onto it to reveal the raw markup).
 
 - colorize this important word
 - now select and colour this whole phrase
diff --git a/development/tests/test-keymaps-vim.vim b/development/tests/test-keymaps-vim.vim
index ef9baeb..45c299d 100644
--- a/development/tests/test-keymaps-vim.vim
+++ b/development/tests/test-keymaps-vim.vim
@@ -545,6 +545,24 @@ call s:record(
       \ 'concealed=' . synconcealed(1, 1)[0]
       \   . ' grp=' . synIDattr(synID(1, stridx(s:cz_l2, 'target') + 1, 1), 'name'))
 
+" A span whose colour Vim can't allocate (rgb()/prose) must be skipped — not
+" abort refresh, not conceal — while a real span afterwards still conceals.
+call s:set_buf(['plain', 'x <span style="color:rgb(1,2,3)">y</span> z', '<span style="color:red">word</span> z'])
+call cursor(1, 1)
+let s:cz_e = ''
+try
+  call nuwiki#colors#refresh()
+catch
+  let s:cz_e = v:exception
+endtry
+call s:record(
+      \ s:cz_e ==# ''
+      \   && synconcealed(3, 1)[0] == 1
+      \   && synconcealed(2, stridx(getline(2), '<span') + 1)[0] == 0 ? 1 : 0,
+      \ 'colorize.skips_unallocatable_colour',
+      \ 'err=' . s:cz_e . ' real=' . synconcealed(3, 1)[0]
+      \   . ' rgb=' . synconcealed(2, stridx(getline(2), '<span') + 1)[0])
+
 " ===== Follow-link resolves + opens in the current window (regression) =====
 " follow_link_or_create() must resolve the definition itself and `:edit` the
 " target in the CURRENT window — including a page that does not exist yet