CraftKit - Custom GUI

    CustomGUI cg = extension.createCustomGUI(null, 45, ChatUtil.formatColorCodes("&aRainbow &fInventory")); // make an empty inventory with 45 slots
    InventoryUtil.fillAll(cg, new ItemBuilder(Material.APPLE)
            .name("&a")
            .build()); // fill the inventory with apples
    cg.addContentCallback(SlotCallback.PREVENT_MODIFY); // make inventory uneditable
    cg.addContentCallback(new SlotCallback() {
        @Override
        public void click(InventoryClickEvent event, Player player, BaseGUI gui) {
            InventoryUtil.fillAll(cg, new ItemBuilder(RandomUtil.pickRandom(MaterialUtil.getStainedGlassPaneTypes())).name("&a").build());
            player.damage(0.0001);
        }
    });
    player.openInventory(cg);