I did something similar in ztkdb, here is the routine. As you can see in the change_col sub, you need to set a 'textstyle' and use -style

############################################################## 
sub select_color{

my $dialogcol = $mw->Toplevel(-bg=>'steelblue');
$dialogcol->overrideredirect(1);
$dialogcol->geometry('+100+100');
$dialogcol->configure(-cursor => 'top_left_arrow');

my @colors = qw(DarkGrey orange pink hotpink red
                peru goldenrod tan gold wheat
                lightyellow yellow khaki lightgreen green
                lightsteelblue turquoise cyan violet plum
                bisque beige snow);

my ($r,$c)=(0,0);
foreach my $color(@colors){
  $dialogcol->Button( -text =>$color,
              -bg=> $color,
              -width=> 12,
              -activebackground=>$color,
               -command => [\&change_col,$color],
                      )->grid(-column => $c, -row => $r);
$c++;
if($c % 5 == 0){$r++;$c = 0};
}

$dialogcol->Button( -text => 'Finished',
              -bg=>'red',
              -activebackground=>'plum',
              -command => sub{$dialogcol->destroy;
                              })->grid(-column=>$c, -row=> $r+1);


sub change_col{
      $info{$key_sel}{'color'} = shift;
      $colorbut->configure(-bg=> $info{$key_sel}{'color'});
               my $textstyle  = $h->ItemStyle('text',
                                -justify => 'center',
                                    -bg => $info{$key_sel}{'color'},
                                    -selectforeground => 'green',
                                   );
     $h->itemConfigure($h->info('selection'), 1,'-style'=>$textstyle);
  }
}
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