off the top of my head, i believe one map is blue, and the other is green. these two maps are independent of each other because they represent two different layers (with the same geometry). so, whatever you explore in one layer has absolutely no effect on what you see in the other layer.
now for A + B, imagine you superimpose whatever information you have from the two independent maps. if you see a blue square, that's because you've explored that square on the blue map already, but not the corresponding square on the green map. likewise, if you see a green square, you've explored it on the green map but not on the blue. if you see cyan (cyan = blue mixed with green, get it?) then you've explored that square in both layers, and so it makes sense to show it in the third color (cyan).
if you want to think about it in terms of "filling up the map" in a certain color, then you want to fill the blue map with blue and the green map with green, and in doing so, you will fill the combined map with cyan.
EDIT: and to see which map you're currently in, check both map A and map B. (ignore map A + B.) the castle you're in should have the flashing thing that tells you where you are. the other map should have no flash. generally speaking castle A has brighter colors and is more "normal" because it represents normal maxim's mind, while castle B has faded colors and is more "twisted" because it represents evil maxim's mind. or something like that.