(Warning: this is another post where I am tired, but sort of have an idea.)
The colour/brightness information is supposed to be sent as a voltage difference; however if the reference voltage is missing or noisy then it could drift up/down causing the colours to invert.
Have you jumpered the pieces of the ground pane (the thick outline that was cut away) together?
If you're making a portable you probably need something to watch it on. (Unless you want to guess what's happening in the game, but I wouldn't advise that) Anyway, this forum is your "Hacking a pocket TV/screen" one-stop solution. Share your experiences and knowledge here.