Display problem on MacBook Pro

Hoping to get some advice on a problem I'm having with my 3 year old MacBook Pro.

Yesterday morning I was using my laptop and closed it once I was done, no problems evident. A couple of hours later when I opened it again, the display screen looked odd, some of the colours were flickering, almost like it was in negative. Most colours are okay but darker shades seem to be affected. Nothing had happened to affect the MacBook, no bumps/knocks, it had just sat on my desk between uses.

I've restarted, restarted in Safe Boot mode, I've cleared suspicious looking files, I've updated the software to the latest version and all to no avail. The toolbar and text on the screen seems to be burning its image into the screen so that on the login screen, I can still see their outline.

I've tried connecting to an external display this morning to see if the problem was the same but the image was fine on the external display.

This is where I get stuck, clearly it isn't a software issue that can be updated away. So my question is whether or not anybody here has experienced this and does anybody have any advice on what the problem might be and the best/easiest/cheapest way to fix it?

Thank you all in advance.