diff --git a/doc/CSApprox.txt b/doc/CSApprox.txt
index e2a30c2..65d79de 100644
--- a/doc/CSApprox.txt
+++ b/doc/CSApprox.txt
@@ -300,6 +300,14 @@ g:CSApprox_hook_{scheme}_post                                 *csapprox-hooks*
           colorscheme named "123 foo_bar-baz456.vim", the hook searched for
           will be, eg, g:CSApprox_hook_foo_barbaz456_post
 
+g:CSApprox_use_showrgb                                *g:CSApprox_use_showrgb*
+    By default, CSApprox will use a built in mapping of color names to values.
+    This optimization greatly helps speed, but means that colors addressed by
+    name might not match up perfectly between gvim (which uses the system's
+    real rgb database) and CSApprox (which uses the builtin database).  To
+    force CSApprox to try the systemwide database first, and only fall back on
+    the builtin database if it isn't available, set this variable non-zero.
+
 g:CSApprox_approximator_function            *g:CSApprox_approximator_function*
     If the default approximation function doesn't work well enough, the user
     (or another author wishing to extend this plugin) can write another
diff --git a/plugin/CSApprox.vim b/plugin/CSApprox.vim
index 1bb96f7..a02986e 100644
--- a/plugin/CSApprox.vim
+++ b/plugin/CSApprox.vim
@@ -294,6 +294,10 @@ let s:rgb_defaults = { "lightred"     : "#FFBBBB",
 " color values (as '#rrggbb').
 function! s:UpdateRgbHash()
   try
+    if !exists("g:CSApprox_use_showrgb") || !g:CSApprox_use_showrgb
+      throw "Not using showrgb"
+    endif
+
     " We want to use the 'showrgb' program, if it's around
     let lines = split(system('showrgb'), '\n')
 
@@ -496,9 +500,9 @@ function! s:SetCtermFromGui(hl)
       let hl.cterm[which] = val
     elseif val =~ '^#\=\x\{6}$'
       let val = substitute(val, '^#', '', '')
-      let r = str2nr(val[0] . val[1], 16)
-      let g = str2nr(val[2] . val[3], 16)
-      let b = str2nr(val[4] . val[5], 16)
+      let r = str2nr(val[0:1], 16)
+      let g = str2nr(val[2:3], 16)
+      let b = str2nr(val[4:5], 16)
       let hl.cterm[which] = g:CSApprox_approximator_function(r, g, b)
       exe 'hi ' . hl.name . ' cterm' . which . '=' . hl.cterm[which]
     else