Basic T-Shirt Designer

I have a very basic 'paint' tshirt designer currently. Please signup and check it out at [login to view URL] then go to settings -> kits. Somebody made me a what I want, but it only works for one layer and is in javascript: [login to view URL] I can easily make it do the same as the javascript one but in PHP and AJAX, however I want to be able to change multiple layer colours, which I'm struggling with as I've never used images in PHP before recently. If you have any questions, please let me know and I'll be happy to answer as best as possible. Many thanks