Here is an example that does what I think you want. It is a hack that swaps colors. The problem with it, is once all colors are swapped to red, it is tricky to restore original bar colors. I used red1, red2, red3 to differentiate and restore. It would seem that you could itemconfigure the progressbar for -colors, to restore original bar colors, but it dosn't seem to work.

I hope it stimulates a solution for you.

#!/usr/bin/perl
use strict;
use Tk;
use Tk::ProgressBar;

our $percent_done = 0;
our $stopped = 0;

my $mw = MainWindow->new(qw/-title Working/);

my $progress = $mw->ProgressBar(
  -width => 20,
  -height => 400,
  -from => 0,
  -to => 100,
  -blocks => 50,
  -colors => [0, 'blue',30,'yellow',70,'orange'],
  -variable => \$percent_done
)->pack;

my $buttonF = $mw->Frame->pack;

my $startB = $buttonF->Button(
  -text => 'start',
  -command => sub {
  $stopped = 0;
  $percent_done = 0;

# hack to restore colors, I use different red shades
# but you could store the information in a hash somehow
# to get back to the original colors
  swapColor($progress, "red1", "blue");
  swapColor($progress, "red2", "yellow");
  swapColor($progress, "red3", "orange");

foreach my $x (1 .. 100) {
 select(undef,undef,undef,.05);
   if ($stopped) {
    swapColor($progress, "blue", "red1");
    swapColor($progress, "yellow", "red2");
    swapColor($progress, "orange", "red3");
  return;
  }
 $percent_done = $x;
 $progress->update;
 }
})->pack(-side => 'left');


my $stopB = $buttonF->Button(
  -text => "stop",
  -command => sub { $stopped = 1; }
 )->pack(-side => 'left');

MainLoop;

sub swapColor {
my ($pb, $oldColor, $newColor) = @_;
  foreach my $tag ($pb->find("all")) {
     print $tag,'->',$pb->itemcget($tag, "-fill"),"\n";
     if ($pb->itemcget($tag, "-fill") eq $oldColor) {
       $pb->itemconfigure($tag, -fill => $newColor);
    }
  }
print "\n\n";
}
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