ImageColorAllocate Problem

Hi there, I am having a problem in ImageColorAllocate function. I am using this function in a loop. After some number of looping the function starts returning -1 value. I am attaching my code below. Can any one help me with this? <?php header("Content-type: image/png"); // create a 100*30 image $im = imagecreate(300, 300); // white background and blue text $bg = imagecolorallocate($im, 255, 255, 255); $j=0; for($i=1;$i<=30;$i++) { $textcolor = imagecolorallocatealpha($im, $i*2, $i*3, $j, 0); // write the string at the top left imagestring($im, 5, 0, $j, "Hello world!", $textcolor); //imagecolordeallocate($im,$textcolor); $j=$j+10; } // output the image imagepng($im,"try.png"); ?> Thanks for reading this far. Kevin.