The only module I know of that might be helpful is Image::Magick.

I don't think you're going to be able to easily do this. JPEG isn't like GIF where there's a single palate of colors that you can just change in one place. You'd basically have to convert the JPEG into another format, change the color in the uncompressed image, and then re-compress back to JPEG. And due to the lossy nature of JPEG compression your going to get a worse (how bad depends on the quality setting used) converted picture.

But yeah, Image::Magick and or GD's the stuff to look into. Or possibly script the Gimp.

I understand. Your reply is useful though. Because of what you say about GIF, isn't it easier if I would use a GIF instead of a JPG? Anything but a .BMP... Thanks for the help!

Detonite

#!/usr/bin/perl
#None of these is fast, mainly because of the overhead of calling Perl
+ 
#code for each pixel. 
use strict;

my $imagefile = shift or die "No file specified\n";

sub rgba2hex {
    sprintf "%02x%02x%02x%02x", map { $_ || 0 } @_;
}

{
    use Imager;
    my %colors;
    my $img = Imager->new();
    $img->open( file => $imagefile ) or die $img->errstr;
    my ( $w, $h ) = ( $img->getwidth, $img->getheight );
    for my $i ( 0 .. $w - 1 ) {
        for my $j ( 0 .. $h - 1 ) {
            my $color = $img->getpixel( x => $i, y => $j );
            my $hcolor = rgba2hex $color->rgba();
            print "$hcolor ";
            $colors{$hcolor}++;
        }
    }

    printf "Imager: Number of colours: %d\n", scalar keys %colors;
}

{
    use GD;
    my %colors;
    my $gd = GD::Image->new($imagefile) or die "GD::Image->new($imagef
+ile)";
    my ( $w, $h ) = $gd->getBounds();
    for my $i ( 0 .. $w - 1 ) {
        for my $j ( 0 .. $h - 1 ) {
            my $index = $gd->getPixel( $i, $j );
            my $hcolor = rgba2hex( $gd->rgb($index), 0 );
            $colors{$hcolor}++;
        }
    }

    printf "GD: Number of colours: %d\n", scalar keys %colors;
}

{
    use Image::Magick;
    my %colors;
    my $img = Image::Magick->new();
    my $rc  = $img->Read($imagefile);
    die $rc if $rc;
    my ( $w, $h ) = $img->Get( 'width', 'height' );
    for my $i ( 0 .. $w - 1 ) {
        for my $j ( 0 .. $h - 1 ) {
            my $color = $img->Get("pixel[$i,$j]");
            my $hcolor = rgba2hex split /,/, $color;
            $colors{$hcolor}++;
        }
    }

    printf "Image::Magick: Number of colours: %d\n", scalar keys %colo
+rs;
}
[download]

I'll give that a shot. Learning's always good. (Except learning what pain is ;) ). Thanks for the help!

1. Background
One of the wonderful features of GIMP is that it all its functionality may be accessed through scripting. So far most of the script programming for Gimp has been done through Scheme through Script-Fu. Unfortunately the Scheme environment Gimp provides is very primitive, e.g. without any reasonable error handling. Furthermore, must users are not familiar with scheme as a language. Some users may therefore prefer to write scripts for the Gimp in Perl.