Mac Book Pro mid 2012 screen colours turned red and purple (similar to rainbow colour)

When I turned on my MacBook earlier, which was in sleep mode for all night, all the colours turned red and purple. The screen pixels were all a kind of red but the computer was working as usual. I turned it off and then back on and the screen was ok. Then I turned it off again for about 6 hours and now the colours are ok. Does anyone know what might be the problem? I am running on a MacBook Pro 13'' Mid 2012 (OS X Maverics) and it is the first time it's done anything like this. I read on Apple Support that maybe the CPU is failing, but when i run the System Diagnostics Test I think it turned out ok. I appreciate all answers.