Very roughly, you burned out the signals that respond to w/e colours it was originally showing, purple and cyan. When it switched to black and white, since those signals were weakened, the inverse colours were overrepresented.

Er, let me try again; it's like you have two guys of equal strength doing tug of war. Before a match, you tire one of the guys out; when it comes time for them to compete, the tired one is losing; even though they're of equal strength, it makes it look like the other one (yellow/brown) is dominant.

The reason you have to stare at the dot is so that the same cones in your eye are taking in the same colour continuously, burning them out so that they're underactive when it's black and white.