Since showrgb also prints out the rgb values you can use the brightness equation to determine foreground font (white or black)... something like:

#!/usr/bin/perl
use strict;
my @colorinfo=<DATA>;
my $i = rand @colorinfo;
my $line=$colorinfo[$i];
$line=~/(\d+)\s+(\d+)\s+(\d+)\s+(.*)$/;
my $brightness=($1*299+$2*587+$3*114)/1000;
my $fg=$brightness>135?"black":"white";
exec("xterm -fg $fg -bg \'$4\' -font 6x13 -ls -sb -sl 1000 &");
__DATA__
199  21 133             medium violet red
176 196 222             light steel blue
102 139 139             paleturquoise4
159 121 238             mediumpurple2
141 182 205             lightskyblue3
  0 238 118             springgreen2
255 160 122             light salmon
154 205  50             yellowgreen
178  58 238             darkorchid2
 69 139 116             aquamarine4
 71  60 139             slateblue4
131 111 255             slateblue1
192 255  62             olivedrab1
139 105  20             goldenrod4
205 155  29             goldenrod3
142 229 238             cadetblue2
[download]

... and so forth. (I just concatenated showrgb to the bottom of my script.) 135 was about the value that worked best for my eyes in terms of a cut off between brightness warranting a black foreground color or a white.