...if you are not stuck with GD, and can use Image::Magick, here is a sample script that will overlay text of any color or transparency, and/or another image

#!/usr/bin/perl
use warnings;
use strict;
use Image::Magick;

#usage: script baseimage overlayimage text

my $image = Image::Magick->new;
my $oimage = Image::Magick->new;

my $rc = $image->Read('1Zen16.jpg');
$rc = $oimage->Read('overlay.png');
my $text = "\nlooking for nirvana\n :-)\n";

$rc = $image->Composite(
     geometry => '+30+30',
     compose=>'Atop',
     image => $oimage,
     opacity => '100%',
     tile  => 0,  
      );

$image->Annotate(pointsize => 36,
         fill      => '#ffcc00ff', #last 2 digits opacity in hex ff=ma
+x
         text      => $text,
         gravity   => 'South' );


$image->Write("$0.png");

exit;
[download]

...also see Image and Text Watermarked Letters and

#!/usr/bin/perl -w
use strict;
use Image::Magick;

=comment
my $text = 'test';
my $im = new Image::Magick;
$im->Read("meteo.jpg");
my $rv = $im->Annotate(
   text   => $text,
   font   => 'C:\\WINDOWS\\Fonts\\Arial.ttf',
   x      => 10,
   y      => 10,
);
die("Unable to annotate image: $rv\n") if "$rv";
print "Content-type: image/jpeg\n\n"; 
binmode STDOUT;
print $im->Write('jpeg:-');
=cut


my $image = Image::Magick->new;
$image->Set(size=>'250x100');
$image->Read('1Zen16.jpg');

my $text = "This is a test";

$image->Annotate(
            pointsize=>40, 
        text=>$text, 
            stroke => '#C0C0C0', # Increase to make stroke more white
            fill => '#505050', #or red etc, Decrease to make fill more
+ black
        gravity => 'center',
        x=> -10, # offsets
        y=> -10  # offsets
        );

print $image->Write("$0.png");
[download]

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