The X11 RGB defintions are found in the xorg.conf file, whose location may be different for different linux distributions, and I don't know where MSWindows puts it.

On my Slackware system, it is in /etc/X11/xorg.conf, and there is an entry in xorg.conf for the RGB definitions

Section "Files"
# The location of the RGB database.  Note, this is the name of the
# file minus the extension (like ".txt" or ".db").  There is normally
# no need to change the default.
    RgbPath     "/usr/share/X11/rgb"
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And my /usr/share/X11/rgb.txt file looks like this:

255 250 250        snow
248 248 255        ghost white
248 248 255        GhostWhite
245 245 245        white smoke
245 245 245        WhiteSmoke
220 220 220        gainsboro
255 250 240        floral white
255 250 240        FloralWhite
253 245 230        old lace
253 245 230        OldLace
250 240 230        linen
250 235 215        antique white
250 235 215        AntiqueWhite
255 239 213        papaya whip
255 239 213        PapayaWhip
255 235 205        blanched almond
255 235 205        BlanchedAlmond
255 228 196        bisque
255 218 185        peach puff
255 218 185        PeachPuff
255 222 173        navajo white
255 222 173        NavajoWhite
255 228 181        moccasin
255 248 220        cornsilk
255 255 240        ivory
255 250 205        lemon chiffon
255 250 205        LemonChiffon
255 245 238        seashell
240 255 240        honeydew
245 255 250        mint cream
245 255 250        MintCream
240 255 255        azure
240 248 255        alice blue
240 248 255        AliceBlue
230 230 250        lavender
255 240 245        lavender blush
255 240 245        LavenderBlush
255 228 225        misty rose
255 228 225        MistyRose
255 255 255        white
  0   0   0        black
 47  79  79        dark slate gray
 47  79  79        DarkSlateGray
 47  79  79        dark slate grey
 47  79  79        DarkSlateGrey
105 105 105        dim gray
105 105 105        DimGray
105 105 105        dim grey
105 105 105        DimGrey
112 128 144        slate gray
112 128 144        SlateGray
112 128 144        slate grey
112 128 144        SlateGrey
119 136 153        light slate gray
119 136 153        LightSlateGray

... etc etc etc
.... etc etc etc
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Somewhere the xorg.conf file got lost. I'm running Debian 6 which does have an rgb.txt file in /etc/X11.

That's not where Tk finds color names. There is a color named DebianRed in that file (215,7,81). Tk quits with:

unknown color name "DebianRed" at /usr/lib/perl5/Tk.pm line 250

The following code:

#!/usr/bin/perl

use strict;
use Tk;


my $mw = MainWindow->new;

my @colors = `cat /etc/X11/rgb.txt`;

my %colorValues;

my ($colorName, $red, $green, $blue, );
shift @colors;      #skip first line
my $colorsRead = 0;
foreach my $colorLine (@colors) {
  chomp $colorLine;
  $colorsRead++;
  if ($colorLine =~ m/(\d+)\s+(\d+)\s+(\d+)\s+(.+)$/) {
    $red = $1;
    $green = $2;
    $blue = $3;
    $colorName = $4;
  } else {
    die "Couldn't parse '$colorLine'\n";
  }
  eval {
    ($red,$green,$blue) = $mw->rgb($colorName);
  };
  if ($@) {
    print "Color named $colorName not found by Tk\n";
  }
}
print "$colorsRead colors read\n";
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Outputs:

Color named DebianRed not found by Tk
753 colors read
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