Wow, that wins the award for the most colorful screen malfunction of the day!
That's great that it doesn't show up on an external screen -- this means that your logic board is good, and that the problem is likely within the realm of the screen components. It doesn't seem that the lighting is flickering or going out (which would point to inverter/inverter cabling/screen), so I suspect the video cable, or the screen itself. The first thing I'd do is pop the topcase off and verify that the video connector is fully seated into the socket on the logic board...it could be something as simple as that. Next, you may need to open the screen casing in order to verify that the video cable isn't getting crimped somewhere, and to also verify there is a solid connection between the video cable and the back of the screen. If none of that does it, I'd replace the video cable, and if that still doesn't fix it, the screen itself may need to be replaced.