To test whether to handle the return value of this function as RGB or a palette index, see either imageistruecolor(), or (for GD < 2.0.1, or PHP 4 < 4.3.2) imagecolorstotal() == 0.
It seems that a number of comments here take many lines to convert RGB or RGBA values to hex strings #AARRGGBB. This is a one-liner in most cases:
$hex = sprintf("#%08X", imagecolorat($im32, $x, $y));
$hex = sprintf("#%08X", $rgb);
$hex = sprintf("#%08X", $argb);
$hex = sprintf("#%02X%06X", $rgba >> 24, $rgba & 0xFF000000);
$hex = sprintf("#00%02X%02X%02X", $r, $g, $b);
$hex = sprintf("#%02X%02X%02X%02X", $a, $r, $g, $b);
For paletted images, it's two lines:
$cols = imagecolorsforindex($imPal, imagecolorat($imPal, $x, $y));
$hex = sprintf("#%08X", $cols['alpha'], $cols['red'], $cols['green'], $cols['blue']);
