It could be that the coffee got into the contacts on the display cable or around the graphics card. The only way to fix it would be to take apart the computer and clean it with cotton swabs and isopropyl alcohol. Make sure everything that is discolored is cleaned up and reseat all the connectors. Also check and make sure all the keys on the keyboard are working, because that is usually the first thing to go during a coffee spill. If you need help taking it apart, you can look at the guides by iFixIt to disassemble, but make sure to take pictures along the way so you remember where everything goes