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);