Original post by: Dan ,
First try connecting an external monitor to see if the problem is present on it. If it is then the graphics chip is having problems. If the external display looks OK, then the internal display is having problems. Here's the IFIXIT guide to replace the display assembly: [guide|17045|MacBook Pro 15" Retina Display Early 2013 Display Assembly Replacement]. Follow Steps 1 thru 4, then jump down to Step 11 to disconnect the display data cable. Carefully inspect the connector surfaces for corrosion. If things look clean then sadly it sounds like the display is damaged.