Darkened screen and distorted colors.

Outside in a hot sunny day will not work. Using a heatgun would work but it is probably not the issue and a heat gun will more than likely burn or warp the board since it isn't a controlled heat it will not stop at the 300-600 degrees that would be needed. I think you may have a loose joint on the a/v connector itself. It is possible that the gpu has a issue but loosing color isn't the first thing that I would put on that kind of fault. (Lockups or no startup would be one that would point to it) Another thing to check would be a bad capacitor or resistor. It also could be a setting like progressive scan or ntsc/pal setting. Not sure since I do not have the unit in front of me. I find these systems at goodwill all the time for around 5 to 15 dollars.