How can I tweak this function to allow dithering?

Question

I wrote the function below. It switches out one color for another. The pictures going in are css sprites with various colors and a white background.

So.. sprite 1 might by blue and sprite 2 might be green.

The function would be run twice to replace the blue + green with whatever colors were required.

/**
 * Changes the color of a graphic.
 * $settings = array (
 *  'icon'
 *  'new_icon'
 *  'old_color' = array
 *  'new_color' = array
 * );
*/
function updateIconColor($settings=array()) {
    // Create Image
    $image = imagecreatefrompng($settings['icon']);

    // Convert True color image to a palatte
    imagetruecolortopalette($image, false, 255);

    // Restore Alpha
    $white = imagecolorclosest($image, 255, 255, 255);
    imagecolortransparent($image, $white);

    // Find + Set color
    $index = imagecolorclosest($image, $settings['old_color'][0],$settings['old_color'][1],$settings['old_color'][2]);
    imagecolorset($image, $index, $settings['new_color'][0], $settings['new_color'][1], $settings['new_color'][2]);

    // Restore Alpha
    imageAlphaBlending($image, true);
    imageSaveAlpha($image, true);

    // Save
    imagepng($image, $settings['new_icon']); // save image as gif
    imagedestroy($image);
}

I need to allow dithering on these images. Is there a way this function can be modified to either cope with dithered images or add dithering itself?

Answer 1

To add dithering in the palette conversion, change:

imagetruecolortopalette($image, false, 255);

to:

imagetruecolortopalette($image, true, 255);