function black() {
        $imgname = "cheetah.png";
        $im = imagecreatefrompng ($imgname);

        $index = imagecolorexact ($im,0,0,0);
        imagecolorset($im,$index,255,0,0);

        $imgname = "cheetah2.png";
        imagepng($im,$imgname);
     }
    //------------------------------------------------------------------

    function white() {
        $imgname = "cheetah.png";
        $im = imagecreatefrompng ($imgname);

        $index = imagecolorexact ($im,255,255,255);
        imagecolorset($im,$index,0,0,0);

        $index = imagecolorexact ($im,0,0,0);
        imagecolorset($im,$index,255,255,255);

        $imgname = "cheetah2.png";
        imagepng($im,$imgname);
    }
     //-----------------------------------------------------------------

    if (isset($_GET['black'])) {
        black();
    }

    if (isset($_GET['white'])) {
        white();
    }

    clearstatcache();

<div class="col-rep">
        <h3>Select a color in pallete and see the result:</h3><br>
        <img class="col-rep-img" src="cheetah.png">
        <a href="?black=true" class="black color"></a>
        <a href="?white=true" class="white color"></a>
        <img class="col-rep-img" src="cheetah2.png?1">
</div>