<?php
function imagecolorallocate2($img, $str) {
 $r = hexdec(substr($str, 1, 2));
 $g = hexdec(substr($str, 3, 2));
 $b = hexdec(substr($str, 5, 2));
 return imagecolorallocate($img, $r, $g, $b);
}
echo imagecolorallocate2($dst_im, '#00FF22');

こうするとか