Calculate difference in percentage between 2 hex colors. Port from Javascript to PHP. For calculating the perception difference, may would be better https://github.com/renasboy/php-color-difference

colormeter.php

<?php

function color_meter($cwith, $ccolor) {
    if (empty($cwith) || empty($ccolor)) return false;
    
    $_cwith  = ($cwith[0] === '#') ? substr($cwith, 1, 7) : $cwith;
    $_ccolor  = ($ccolor[0] === '#') ? substr($ccolor, 1, 7) : $ccolor;

    $_r = intval(substr($_cwith, 0, 2), 16);
    $_g = intval(substr($_cwith, 2, 2), 16);
    $_b = intval(substr($_cwith, 4, 2), 16);

    $__r = intval(substr($_ccolor, 0, 2), 16);
    $__g = intval(substr($_ccolor, 2, 2), 16);
    $__b = intval(substr($_ccolor, 4, 2), 16);

    $p1 = ($_r / 255) * 100;
    $p2 = ($_g / 255) * 100;
    $p3 = ($_b / 255) * 100;

    $perc1 = round(($p1 + $p2 + $p3) / 3);
    
    $p1 = ($__r / 255) * 100;
    $p2 = ($__g / 255) * 100;
    $p3 = ($__b / 255) * 100;
    
    $perc2 = round(($p1 + $p2 + $p3) / 3);
    return abs($perc1 - $perc2);   
}