Mix & Match Discount

I want to have mulitbuy discount on my Opencart shop for products that come in different colours or designs. I am finding if I use options, customers can put too many of one option in the cart and it will not tell them until they check out. If I put them on as separate products I cannot get them to multibuy. Is there a way to do this or a module. E.G. I have about 20 designs of Fantasy Mugs that are £4.99 each. I want to do them for £4 if you buy 6, they can be six different designs or 2 of each of the 3 cat designs for instance.