Your best bet is to repair it with the original grout if at all possible. And sell them sealer and seal the entire floor. Some customers may have a little grout left by the tile setter for said repairs. We generally leave them a small bag for just that purpose. Otherwise pickup a few grout charts from your local supplier and match that grout. You may check with your customer to see if they have their invoice from the floor install.. highly unlikely but sometimes they come through for you. Often the brand of grout will be stated. And if the homeowner performed the job themselves, they will often know what brand grout they used. Just my 0.02 cents.