This is always a bad situation for the buyer and the seller. Seller wants to do as cheap of a roof as he can. Buyer will have to live with the result.
I'd try to turn it into a selling point. "Choose your own shingle color" or add the offer to upgrade to a hail resistant shingle or some other extra. Keep the new roof in the price. Let them pick it out and get it installed before they take possession so it can be financed in the house mortgage.