Correct me if I'm wrong, but I have to agree with NATO. If you can take a screenshot of it then it is likely not a fault with the screen or any connection to it. This means that it has been generated by the computer before being sent to the screen. This is similar with inverting your screen; it is not possible to take a picture when the colours of your screen have been inverted because the card is changing the colours rather than the computer itself.
Could be a virus (yes, they do sometimes happen), or more likely a hardware problem in the machine.
Get it sorted at an Apple retailer, don't even think about opening up that beautiful box because once you do, you probably won't be able to fix it and any waranty will be void.
Good luck.