Most shirts, especially red ones will show this discoloration after pressing. It will normalize after about an hour.
You can minimize this some by placing a teflon sheet between the garment/transfer and the heat platen. If you don't have a teflon sheet, and you need to get these shirts done pronto, go to the baking aisle of your grocery store and buy some parchment paper to use until you can get a teflon sheet.
If it is a white shirt that has scorched, there could be several reasons. It could be the fabric itself -- is it something other than cotton or a cotton poly blend? Could your heat be too high? Are these shirts purchased at the retail level? If so, they may have a sizing on the fabric that's scorching when heated.