Could need degaussing or purity convergence, but you might first try rotating your game 90 degrees one way or the other. I have a Space Duel which had pinkish color in the upper right corner where the red score is. Tried degaussing it and it didn't help, but when I faced the game west rather than north, the pinkish color turned to red like it's supposed to be.
You might also be experiencing a magnetic field from a nearby game, so putting it next to a different game could help too.