A useful function
Using these functions one could write a get_color() function that will do everything possible to resolve the best possible match for the desired color: 

<?php
    function get_color($img, $red, $green, $blue) {
        $color = imagecolorexact($img, $red, $green, $blue);
      
        if($color == -1) {
            $color = imagecolorallocate($img, $red, $green, $blue);
      
            if($color == -1) {
                 $color = imagecolorclosest($img, $red, $green, $blue);
            }
        }
      
        return $color;
    }
?>
However why bother re-inventing the wheel?
This is exactly what imagecolorresolve() is for!

imagecolorresolve($img_r, $red, $green, $blue);