Color palette higher than 255

Question

For my project, I'm able to print textures on objects. As soon I use nicer textures that use a color palette higher than 256 it will turn black or invisible...

Is anyone able to help me with this issue? Right now this is my code to transfer the .png into a useable texture:

public static Background getIndexedImage(int id, File file) throws IOException {

    BufferedImage image = ImageIO.read(file);

    List<Integer> paletteList = new LinkedList<>();
    paletteList.add(0);
    int width = image.getWidth();
    int height = image.getHeight();
    byte[] pixels = new byte[width * height];
    for (int x = 0; x < width; x++) {
        for (int y = 0; y < height; y++) {
            int rgb = image.getRGB(x, y);
            int red = rgb >> 16 & 0xff;
            int green = rgb >> 8 & 0xff;
            int blue = rgb & 0xff;
            int alpha = rgb & 0xff;
            rgb = red << 16 | green << 8 | blue;
            if (alpha == 255) {
                rgb = 0;
            }
            int index = paletteList.indexOf(rgb);
            if (index == -1) {

                if (paletteList.size() < 256) {
                    index = paletteList.size();
                    paletteList.add(rgb);
                } else {
                    throw new IllegalArgumentException("The target image has more than 255 color in the palette "+id);
                }
            }
            pixels[x + y * width] = (byte) index;
        }
    }
    int[] palette = new int[paletteList.size()];

    final AtomicInteger index = new AtomicInteger(0);


    for (int pallet = 0; pallet < paletteList.size(); pallet++) {
        palette[index.getAndIncrement()] = paletteList.get(pallet);
    }
    return new Background(width, height, palette, pixels);
}