Here's a handy function that finds the first occurrence of a specific pixel. You can set the tolerance of the color you are looking for, or set it to 0 if want an exact match
<?php
function findPixel($img, $r, $g, $b, $tolerance=5)
{
$original_                 = new Imagick($img);
$height                    = 0;
$width                    = 0;
    list($width, $height)    = getimagesize($img);
$matrix_org                = array();
$matrix_mrk                = array();
    for( $x = 0 ; $x < $width ; $x++){
$matrix_org[$x]            = array();
$matrix_mrk[$x]            = array();
    }
    for( $x = 0 ; $x < $width ; $x++ )
    {
        for( $y = 0 ; $y < $height ; $y++ ){
$matrix_org[$x][$y]                = $original_->getImagePixelColor($x, $y)->getColorAsString();
$colors                         = preg_replace('/[^-,0-9+$]/', '', $matrix_org[$x][$y]); 
$colors                            = explode(',', $colors);
$r_org                            = $colors[0];
$g_org                            = $colors[1];
$b_org                            = $colors[2];
            if(     ( $r <= ($r_org+$tolerance) && $r >= ($r_org - $tolerance) ) 
                &&  ( $g <= ($g_org+$tolerance) && $g >= ($g_org - $tolerance) ) 
                &&  ( $b <= ($b_org+$tolerance) && $b >= ($b_org - $tolerance) ) )
            {
                return array( $x, $y );
            }
        }
    }
    return false;
}
?>