A way to get each time an answer :
<?php
function imagegetcolor($im, $r, $g, $b) {
$c=imagecolorexact($im, $r, $g, $b);
if ($c!=-1) return $c;
$c=imagecolorallocate($im, $r, $g, $b);
if ($c!=-1) return $c;
return imagecolorclosest($im, $r, $g, $b);
}
?>
If the *exact* color is found in the image, it will be returned. If we don't have the exact color, we try to allocate it. If we can't allocate it, we return the closest color in the image.